TINYGL: Fixed support to RGB textures.

This PR contains a simple fix to tinyGL: it is now possible to create empty RGB textures.
This also fixes an issue with myst3 crashing when subtitles were enabled.
Isn't this easier to read as:
else if ((format != TGL_RGBA && format != TGL_RGB && format != TGL_BGR) || type != TGL_UNSIGNED_BYTE)) ?