gallium/vl: compositor fixes

This is a series of cleanup and fixes for gallium/vl.

This MR fixes:

• remove garbage lines/columns display when buffer width/height is bigger than the texture real width/height (this is the same fix than the one in !5387 (closed))
• display of only a portion of the source when using vl_compositor_gfx
• rotation, rgb -> yuv when pipe_cs_composit_supported = true

The last commit is only for testing and I don't plan to merge it:

• VL_MESA_CS_DISABLE=1 disable vl_compositor_cs
• VL_MESA_ROTATE=x force layer rotation to x° (x can be: 0, 90, 180, 270)
• VL_MESA=srcwxsrcy+sx+sy:dstwxdsty+dx+dy: all values are floats in the [0:1] interval. eg: VL_MESA=0.5x0.5+0.24+0:0.5x0.5+0.1+0.3 mpv --vo=vaapi title.jpg produces the following image:

cc @thongthai @leoliu