Support loading multiple EGL platforms in a single process

Change EGLModule to have an EGLDriver per supported platform and cache
based on EGLModule-platform pair rather than just EGLModule.
nouveau: Support DRI Image 7 extension
Merge branch 'egl-platform-mir' into egl-platform-mir-use-dma-buf

Fix use of mir_egl_native_display_is_valid