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.
We do not want to build GL-based importers when GTK GL windowing support is missing,
which might happen because of missing deps or on unsupported platform
No need to auto build rawimporter if we are building gluploader as it
will be always loaded on startup, but never used.
Skip the build of it when its meson option is set to "auto".
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.
