plugin: Add GL uploader

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.

meson_options.txt

@@ -24,6 +24,11 @@ option('glimporter',
   value: 'auto',
   description: 'Build GL memory importer for clappersink'
 )
+option('gluploader',
+  type: 'feature',
+  value: 'auto',
+  description: 'Build GL uploader for clappersink'
+)
 
 option('devel-checks',
   type: 'boolean',