Heretic and Hexen: check for valid, existing characters in font drawing functions

[JNechaevsky]

This should fix possible crashes in cases like:

• Using lowercased designation (am/pm instead of AM/PM) in savegame time. This was fixed by not drawing lowercase characters.
• Lowecased text can be used for both big and small fonts, probably good for compatibility with mods.
• Unknown characters (not existing in font graphics) are still replaced with spaces.

[JNechaevsky]

@fabiangreffrath, how about this approach? There is definitely some potential for readability improvement, like using macros instead of bare ACSII indexes (const int's probably won't be happy about [\\]^_'{|}~ charaters) and comments improvement. But in general, is it acceptable and should I proceed?

[JNechaevsky]

Just a moment, just a moment, there is still Hexen to take care.

[JNechaevsky]

Allright, it's all done! Hexen is using MN_DrTextA() for automap strings drawing, that is now safe for CaSe SeNsItivItY. Intermission and finale texts are using toupper() correction and all such texts are in lowercase right in Hexen.wad (CLUS1-3MSG and WIN1-3MSG lumps). In-game ACS prints are obliviously MN_DrTextA() and MN_DrTextAYellow()'s.

Thank you very much for recommendations, it's much cleaner and readable now than my prototype I wrote in the morning, almost right after getting from bed. 😀

Ah yes, and of course, initial issue fixed and time is now appearing as it should for lowercase am/pm: