Support for old OpenGL versions

[reivilibre]

Hello there,

For my project (I want to run on a PinePhone with Mali 400 GPU), I needed to be able to support OpenGL ES 2 (maximum GLSL version 1.20).

This library makes use of instanced rendering (which doesn't exist on GL ES 2) and gl_VertexID (and some other syntactic modern GLSL style), which I've had to work around.

I have, for the time being, pieced together the changes needed to downgrade OpenGL and GLSL and made a fork ( https://bics.ga/reivilibre/glow_glyph_compat ).

However, it would be nice to make glow_glyph itself have a fallback mode for these old versions. I would be interested to hear your thoughts about this.

Thanks for your libraries!