ui/gtk-gl-area: create the requested GL context version

Since about 2018 virglrenderer (commit fa835b0f88 "vrend: don't
hardcode context version") tries to open the highest available GL
context version. This is done by creating the known GL context
versions from the highest to the lowest until (*create_gl_context)
returns a context != NULL.

This does not work properly with
the current QEMU gd_gl_area_create_context() function, because
gdk_gl_context_realize() on Wayland creates a version 3.0 legacy
context if the requested GL context version can't be created.

In order for virglrenderer to find the highest available GL
context version, return NULL if the created context version is
lower than the requested version.

This fixes the following error:
QEMU started with -device virtio-vga-gl -display gtk,gl=on.
Under Wayland, the guest window remains black and the following
information can be seen on the host.

gl_version 30 - compat profile
(qemu:5978): Gdk-WARNING **: 16:19:01.533:
  gdk_gl_context_set_required_version
  - GL context versions less than 3.2 are not supported.

(qemu:5978): Gdk-WARNING **: 16:19:01.537:
  gdk_gl_context_set_required_version -
  GL context versions less than 3.2 are not supported.

(qemu:5978): Gdk-WARNING **: 16:19:01.554:
  gdk_gl_context_set_required_version -
  GL context versions less than 3.2 are not supported.
vrend_renderer_fill_caps: Entering with stale GL error: 1282

To reproduce this error, an OpenGL driver is required on the host
that doesn't have the latest OpenGL extensions fully implemented.
An example for this is the Intel i965 driver on a Haswell processor.

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20220605085131.7711-2-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>