backend: Add fallback to gtksink
This implements an automatic fallback from gtkglsink to gtksink in the following cases:
- When the environment variable
TOTEM_USE_GST_GTKSINK=1
is specified. - When the GTK GL context cannot be initialized due to insufficient GL driver.
- When a software GL rasteriser (llvmpipe or softpipe) is used, because gtksink performs better in this case.
Edited by Balló György