gl: add support for A420 conversion

gl: add support for A420 conversion

A420 is a four planar format similar to I420 but with an extra buffer
for alpha values.

A common use of the gl stack is for GPU format conversions using
shaders, in which case one can use glupload, glcolorconvert and
gldownload elements to upload their buffer to the GPU context, perform
the conversion on the GPU itself and then retrieve the data to CPU
context.

A420 was not supported. This patch adds said support mainly by adding
the corresponding shader to perform the conversion and updating the
supported caps.

Both A420->RGBA and RGBA->A420 conversions are supported.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1153>