Implement EGL_MESA_gl_texture_cubemap_image
See https://github.com/KhronosGroup/EGL-Registry/pull/165
Depends on !18673 (merged) and !18848 (merged)
Tests piglit!717
See https://github.com/KhronosGroup/EGL-Registry/pull/165
Depends on !18673 (merged) and !18848 (merged)
Tests piglit!717