Implement software cursor in OpenGL backend

This change is needed for Wayland screen recording apps to work
correctly. With this change the cursor is actually visible using GBM
buffer passing protocol.

Previously OpenGL backend did not support software cursor. If launching
in DRM/OpenGL mode with flicked on software cursor it only rendered
scene, but not the cursor image.

Differential Revision: https://phabricator.kde.org/D6186