Review and clean up `#version` directive handling

[c-lipka]

In preparation for v3.8, handling of the `#version´ directive needs thorough investigation, and possibly fixing. In the end, the following criteria should be met:

• Legacy scenes and include files starting with a #version directive should work fine without warnings.
• Legacy scenes without a #version directive should parse as if they specified #version 3.62

A few complications need to be taken into consideration, that were not properly addressed in v3.7:

• There must be a reliable, warning-free and backward compatible way for include files to change the effective scene language version, yet revert back to the previous effective version after their job is done. The following pattern has been in use for this purpose for such a long time that include files making use of it must be successful in restoring the effective version:

  #local OldVersion = version;
  #version 3.5;
  //...
  #version OldVersion;

  Most notably, this must work regardless of whether the main scene file had already specified a #version directive or not. The latter case is something v3.7 failed to handle properly. (Note however that, in this context, the above pattern need not necessarily work per se; instead, a fully automatic mechanism, that always restores the effective version at the end of an include file no matter what, would also fit the bill.) When implementing whatever solution we choose, it must be ensured that the same mechanism also works as expected for files included via INI option rather than #include directive.

• There must be a reliable, warning-free and backward compatible way for scenes to probe the maximum supported scene language version. Note that although undesirable, it would be acceptable for legacy scenes to be presented with wrong information, and therefore misidentify v3.8 as v3.7 or even v3.6.2. Also note that in order to satisfy v3.7's requirement for a scene to start with a #version statement, while also remaining able to probe the supported version, a version-adaptive scene currently must start with #version version; to silence v3.7 warnings. So whatever solution we choose for v3.8, it must accept that idiom.

[c-lipka]

v3.8-alpha has been trying to solve this issue using the following approach:

• Instead of having `version´ evaluate to the maximum supported scene language version, it has that variable evaluate to the effective supported scene language version.
• As an exception, when encountered in the #version version; idiom at the start of the main scene file, the variable would evaluate to the maximum supported version.

However, this approach is frought with special handling of various edge cases, and is therefore a rather fragile solution.

[c-lipka]

I hereby propose the following approach:

• Revert back to the v3.7 handling, i.e. have version evaluate to the maximum supported language version.
• Upon including any file, remember whether the effective scene version is still set to the implicit default (i.e. has not been deliberately overridden).
• Upon reaching the end of any include file, set the effective scene language version back to the implicit default if
  • that had been the state before including the file (otherwise the include file's own mechanisms would have had a chance to pick up the explicit choice, and deal with it accordingly), and
  • the effective scene language version at the end of the include file is equal to the maximum supported version (i.e. the value the include file would have determined upon probing the version variable).

Alternatively, we could just memorize the complete state of language version selection (effective version and whether implicit by default or explicit by choice) at the beginning of any include file, and revert it at the end unconditionally. However, that would preclude include files from deliberately changing what language verison the main scene file will be using. The proposed solution would prevent such tampering only if:

• the main scene hasn't decided what version to use; and
• the include file decides to force the version to v3.8.

[c-lipka]

• Upon reaching the end of any include file, set the effective scene language version back to the implicit default if [...]

Caveat: The solution does involve special case handling, so might be difficult to explain to users.

Another alternative might be to not actively revert the effective language state, and instead just print a warning (which we should do either way) informing the user that the include file was the first to set a #version, and that this is troublesome and may cause undesired effects, and to suppress this warning they may want to add #version 3.62; to the top of your main scene file, or use the Version=3.62 or +mv3.62 INI file / command line option.

This would also have the benefit of being 100% compatible to v3.7 behavior in this regard.

[c-lipka]

I have decided to leave #version handling as it currently is in v3.8, except that specifying #version 3.8 in the main scene file, after having failed to start the scene with any #version directive whatsoever, will not cause an outright parse error. (The rationale for this change is that the current implementation of this error fails to account for certain fringe cases, in which it would trigger by mistake.)