fs: Exclude FolderWatcher implementation for non-Mac platforms
gfx: Move OpenGL includes to opengl.hpp

This makes it easier to support other platforms and to include extension/stub layers
gfx: Adjust treatment of compressed format to support more than just BCn

Not all compressed formats are 4x4 block-compressed; PVRTC and ASTC have different rules of computing the image sizes.
gfx: Add Texture::Format_D16