virgl: Fix texture transfers by using a staging resource

virgl: Fix texture transfers by using a staging resource

This commit fixes the following flaws in the implementation:

* when a resource was re-allocated, the guest side storage
  was also allocated
* when a source needs a readback before being written to, then
  the call would go through vws->transfer_get, thereby bypassing the
  staging resource, and this would fail on the host, because no
  the allocated IOV was too small (just one byte)
* if the texture write would need neither flush nor readback, the
  old code path would be used expecting that guest side backing stogage
  for the texture.

v2: - actually do a readback to the stageing resource when it is required
    - fix typo (Lepton)

v3: Don't use stageing transfers if the host can't read back the data
    by rendering to an FBO or calling getTexImage, because in this case
    we rely on the IOV to hold the date.

v4: Also don't use staging transfers if the format is no readback
    format. Otherwise we have to deal with the resolve blit, and
    this is currently not working correctly.

v5: add a new flag that indicates whether non-renderable textures can
    be read back (either via glGetTexImage or GBM)

v6: Restrict the use of staging texture transfers to textures that can
    be read back, and on GLES also if the they are bound to scanout and
    the host uses minigbm to allocate such textures.
    For that replace the flag indicating the capability to read back
    non-renderable textures with a cap that indicates whether scanout
    textures can be read back.

v7: update virglrenderer version in the CI

v8: update use of stageing (Chia-I)

v9: remove superflous check and assignment (Chia-I)

v10: disable stageing textures for arrays with stencil format. This is a
     workaround for failures of the CI.

Fixes: cdc480585c9be368ddfdc33e2eb73e3582f25fe7
    virgl/drm: New optimization for uploading textures

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14495>