ANGLE extensions are only provided by Google's Almost Native Graphics Layer
Engine (ANGLE) implementation. Therefore they do not seem too useful for
Mutter.
The reason to drop GL_ANGLE_framebuffer_blit support is that it has more
limitations compared to the glBlitFramebuffer in GL_EXT_framebuffer_blit,
GL_NV_framebuffer_bit, OpenGL 3.0 and OpenGL ES 3.0. Most importantly, the
ANGLE version cannot flip the image while copying, which limits
_cogl_blit_framebuffer to only off-screen <-> off-screen copies. Follow-up work
will need off-screen <-> on-screen copies.
Instead of adding yet more capability flags to Cogl, dropping ANGLE support
seems appropriate.
The NV extension is added to the list of glBlitFramebuffer providers because it
provides the same support as ANGLE and more.
Likewise OpenGL ES 3.0 is added to the list of glBlitFramebuffer providers
because e.g. Mesa GLES implementation usually provides it and that makes it
widely available, again surpassing the ANGLE supported features.
Follow-up patches will lift some of the Cogl assumptions of what
glBlitFramebuffer cannot do.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/615
