plugin: Rework code to use new GL context handler

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.

lib/gst/plugin/importers/meson.build

@@ -1,20 +1,9 @@
-gst_clapper_gl_base_importer_dep = dependency('', required: false)
-
 all_importers = [
   'glimporter',
   'gluploader',
   'rawimporter',
 ]
 
-build_glbase = (
-  not get_option('glimporter').disabled()
-  or not get_option('gluploader').disabled()
-)
-gl_support_required = (
-  get_option('glimporter').enabled()
-  or get_option('gluploader').enabled()
-)
-
 # We cannot build any importers without sink that they depend on
 if not gst_clapper_sink_dep.found()
   foreach imp : all_importers
@@ -24,89 +13,16 @@ if not gst_clapper_sink_dep.found()
   endforeach
 endif
 
-gst_plugin_gl_base_deps = [gst_clapper_sink_dep, gstgl_dep, gstglproto_dep]
-have_gtk_gl_windowing = false
-
-if gst_gl_have_window_x11 and (gst_gl_have_platform_egl or gst_gl_have_platform_glx)
-  gtk_x11_dep = dependency('gtk4-x11', required: false)
-  if gtk_x11_dep.found()
-    gst_plugin_gl_base_deps += gtk_x11_dep
-    if gst_gl_have_platform_glx
-      gst_plugin_gl_base_deps += gstglx11_dep
-    endif
-    have_gtk_gl_windowing = true
-  endif
-endif
-
-if gst_gl_have_window_wayland and gst_gl_have_platform_egl
-  gtk_wayland_dep = dependency('gtk4-wayland', required: false)
-  if gtk_wayland_dep.found()
-    gst_plugin_gl_base_deps += [gtk_wayland_dep, gstglwayland_dep]
-    have_gtk_gl_windowing = true
-  endif
-endif
-
-if gst_gl_have_window_win32 and (gst_gl_have_platform_egl or gst_gl_have_platform_wgl)
-  gtk_win32_dep = dependency('gtk4-win32', required: false)
-  if gtk_win32_dep.found()
-    gst_plugin_gl_base_deps += gtk_win32_dep
-    have_gtk_gl_windowing = true
-  endif
-endif
-
-if gst_gl_have_window_cocoa and gst_gl_have_platform_cgl
-  gtk_macos_dep = dependency('gtk4-macos', required: false)
-  if gtk_macos_dep.found()
-    gst_plugin_gl_base_deps += gtk_macos_dep
-    have_gtk_gl_windowing = true
-  endif
-endif
-
-if not have_gtk_gl_windowing
-  if gl_support_required
-    error('GL-based importer was enabled, but support for current GL windowing is missing')
-  endif
-  build_glbase = false
-endif
-
-if gst_gl_have_platform_egl
-  gst_plugin_gl_base_deps += gstglegl_dep
-endif
-
-foreach dep : gst_plugin_gl_base_deps
-  if not dep.found()
-    if gl_support_required
-      error('GL-based importer was enabled, but required dependencies were not found')
-    endif
-    build_glbase = false
-  endif
-endforeach
-
-if build_glbase
-  gst_clapper_gl_base_importer_dep = declare_dependency(
-    link_with: library('gstclapperglbaseimporter',
-      'gstclapperglbaseimporter.c',
-      c_args: gst_clapper_plugin_args,
-      include_directories: configinc,
-      dependencies: gst_plugin_gl_base_deps,
-      version: libversion,
-      install: true,
-    ),
-    include_directories: configinc,
-    dependencies: gst_plugin_gl_base_deps,
-  )
-endif
-
 build_glimporter = (
   not get_option('glimporter').disabled()
-  and gst_clapper_gl_base_importer_dep.found()
+  and gst_clapper_gl_ch_dep.found()
 )
 
 if build_glimporter
   library(
     'gstclapperglimporter',
     'gstclapperglimporter.c',
-    dependencies: gst_clapper_gl_base_importer_dep,
+    dependencies: gst_clapper_gl_ch_dep,
     include_directories: configinc,
     c_args: gst_clapper_plugin_args,
     install: true,
@@ -116,14 +32,14 @@ endif
 
 build_gluploader = (
   not get_option('gluploader').disabled()
-  and gst_clapper_gl_base_importer_dep.found()
+  and gst_clapper_gl_ch_dep.found()
 )
 
 if build_gluploader
   library(
     'gstclappergluploader',
     'gstclappergluploader.c',
-    dependencies: gst_clapper_gl_base_importer_dep,
+    dependencies: gst_clapper_gl_ch_dep,
     include_directories: configinc,
     c_args: gst_clapper_plugin_args,
     install: true,