GLTexture: Add GL_RGB format support
When creating a GdkGlTexture
from a GLTexture I get with QEMU, I was getting that GL_RGB
format is unsupported. Per the docs at https://registry.khronos.org/OpenGL-Refpages/gl4/html/glTexImage2D.xhtml & if my understanding is correct. It should be safe to map GL_RGB
to GDK_MEMORY_R8G8B8