Instead figuring out what kind of pool we should make just use the upstream provided
functionality that comes as part of GLUpload API.
Fixes uploads with GstVideoGLTextureUploadMeta, which legacy VAAPI plugin uses on GLX.
Make GL base importer class and all its subclasses that access parent props,
use a different mutex lock then the main importer class.
This way main importer class can continue processing buffers without waiting for
GL-based subclasses, as most GL operations (e.g. upload) are time consuming.
Add "clappergluploader" module which uses "glupload" and "glcolorconvert" internally,
allowing either uploading RAW system memory into GPU with GL colorspace conversion or
importing DMABufs into GL memory and making a GdkTexture out of them at one go.
