gsk: Add GSK_RENDERING_MODE=full-redraw

gsk: Add GSK_RENDERING_MODE=full-redraw

Forces a full redraw every frame.
This is done generically, so it's supported on every renderer.

For widget-factory first page (with the spinner spinning and progressbar
pulsing), I get these numbers per frame:

action                  clipped         full redraw
snapshot                   0ms           7-10ms
cairo rendering            0ms          10-15ms
Vulkan rendering         3-5ms          18-20ms
Vulkan expected *          0ms            1-2ms
GL rendering            unsupported     55-62ms

* expected means disabling rendering of unsupported render nodes,
instead of doing fallback drawing. So it overestimates the performance,
because borders and box-shadows are disabled.