sdl2: Support all virtio-gpu formats

sdl2: Support all virtio-gpu formats

There are some 2D resource formats that can be used through virtio-gpu,
but which are not supported by SDL2 when used for a scanout; these are
all alpha-channel formats and also XBGR (RGBX in non-BE pixman).

Add these formats in the switch converting pixman to SDL format
constants so a guest cannot crash the VM by triggering the
g_assert_not_reached() with an unsupported format.

Signed-off-by: Max Reitz <[email protected]>
Message-id: 20181008185013[email protected]

[ kraxel: also update sdl2_2d_check_format() ]

Signed-off-by: Gerd Hoffmann <[email protected]>