virgl: Add tweak to apply a swizzle when drawing/blitting to a emulated BGRA texture

virgl: Add tweak to apply a swizzle when drawing/blitting to a emulated BGRA texture

With Qemu this final swizzle is not needed, but with vtest it is, i.e. it depends on
how a program using virglrenderer uses the surface that is rendered to, hence
a tweak is added.

v2: Update description and fix spelling (Emil)
v3: Send tweak as binary value instead of using strings (Gurchetan)
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>