backend: Disable vaapi at the root

Changes in gst-vaapi, vaapi, and the vaapi driver, combined with clutter-gst's bad integration with those means that things regularly break, and totem isn't able to display anything properly.

Until we switch the display sink away from clutter-gst and clutter-gtk we need to disable vaapi to get anything on screen.

Fixes: Shader compilation failed: 0:91(21): warning: cogl_layer0' used uninitialized 0:104(21): warning: cogl_layer1' used uninitialized 0:117(21): warning: `cogl_layer2' used uninitialized 0:128(16): error: no function with name 'clutter_gst_sample_video3' Failed to link GLSL program: error: linking with uncompiled/unspecialized shader