Better shader support

[filloax]

Mod shaders in AB+ are very hard to work with, and are missing important features. Of note:

• Being unable to use GLSL features after the very first version, including arrays and others, in all GPUs due to the version line being unusable (I assume because the XML code gets injected after line 1 in the actual shader code)
• Being unable to load images and game data to mask out effects, apply them only on certain entities/stage portions, etc
• Probably being more laggy than vanilla ones? That one is probably as much on my side as the game's, though

Another useful but less important feature would be being able to 100% disable them via code (instead of still loading them and using a GLSL variable to do so), so an option could be added for users with less powerful machines, else they seem to still have a performance impact

(Note: I am not too proficient with shaders in other contexts myself, so if someone more knowledgeable can add on this please do)

[TazTxUK]

Getting access to other shader types (namely geometry) would be fantastic also, as draw heavy mods can struggle sometimes, especially with older computers.