Create a GL context handler subclass of recently added context handler base class and
rework code to use it. This simplifies a lot of things, like: switching of importers
at runtime, handling context queries, sharing data between importers, etc.
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.
