Add const modifier to drawing API

[iacore]

there may be others than can be changed so

[raysan5]

@iacore Sorry, no plans to implement this change for now.

[iacore]

const T* is used in the header file.

for example, here

RLAPI Image GenImageFontAtlas(const GlyphInfo *glyphs, Rectangle **glyphRecs, int glyphCount, int fontSize, int padding, int packMethod); // Generate image font atlas using chars info

One Zig bindings generator relies on this info.

[iacore]

@raysan5 please reconsider.

[raysan5]

@iacore Appreciate your insistence but not for now.