There are no longer any importers of package backend outside of
gioui.org/gpu. Move it internally, and rename it to the slightly more
specific "driver" while we're at it.
Signed-off-by: Elias Naur <mail@eliasnaur.com>
The package app/internal/d3d11 now contains only the GPU backend on
Direct3D. Move it below package gpu to reflect that.
Signed-off-by: Elias Naur <mail@eliasnaur.com>