Implement texture atlas

[Drwalin]

Implement texture atlas with dynamic downsampling of too big textures in case of lack of memory for new textures.
In case of lack of free memory within texture atlas, the one of the biggest textures should be first downsampled and only thereafter new textures can be placed inside texture atlas.

Supported textures should be in size of power of 2 and square (optionally any size). Unique textures could be placed outside of an atlas.

Use glTextureView from GL_ARB_texture_view to selec slices of texture array (for use with i.e. ImGUI). This approach requires texture storage to be immutable (generated with glTex*Storage*D instead of glTexImage*D).