The sizing policy allows selecting between the current behavior,
which deforms the texture to fill the width and height of the
pad; and a new 'keep-aspect-ratio' sizing policy, which fits the
texture within the rectangle respecting its original aspect ratio.
The reason for this is that this allows avoiding extra elements
in the pipeline, and reduces the number of buffer passing through
the pipeline.
Most of this code is a direct port of the sizing policy handling
of the compositor element, except it is adapted to operate on GL
texture coordinates through the projection matrix.
<https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3760>
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3760>
Handling mouse navigation events in glvideomixer element, if no
pixel-aspect-ratio info in the caps, an assertion error is produced
inside gst_util_fraction_multiply because default denominator is zero.
Error fixed:
```
(gst-launch-1.0:102654): GStreamer-CRITICAL **: 00:47:51.598: gst_util_fraction_multiply: assertion 'b_d != 0' failed
```
Simple pipeline to reproduce the issue:
```
gst-launch-1.0 -v glvideomixer name=mix ! glimagesinkelement gltestsrc ! mix.sink_0
```
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3766>
