Document and verify expected return values for make_current() callback
The values retuned by Qemu's make_current() callback varies based on used GL backend. In case of SDL GLX, Qemu returns 0 on success, while for EGL backends Qemu confusingly returns 1. Document the expected return values and enforce the new requirement for the callbacks v4, preserving compatibility for older virglrenderer users implementing v1 callbacks.