Refactor GL rendering to prepare for GTK4

- Remove FlGLArea - we can just use GtkGLArea directly
- Stop using gdk_cairo_draw_from_gl, it's not available in GTK4.
- Rename FlRendererGL to FlRendererGdk, this was implying that FlRenderer could
  be something other than OpenGL which is not the case currently.
- Remove unnecessary virtual methods in FlRenderer, just implement them as
  standard methods.

Author: robert-ancell

ci/licenses_golden/licenses_flutter

@@ -10926,8 +10926,8 @@ ORIGIN: ../../../flutter/shell/platform/linux/fl_plugin_registrar_test.cc + ../.
 ORIGIN: ../../../flutter/shell/platform/linux/fl_plugin_registry.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/linux/fl_renderer.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/linux/fl_renderer.h + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/shell/platform/linux/fl_renderer_gl.cc + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/shell/platform/linux/fl_renderer_gl.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/linux/fl_renderer_gdk.cc + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/linux/fl_renderer_gdk.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/linux/fl_renderer_headless.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/linux/fl_renderer_headless.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/linux/fl_scrolling_manager.cc + ../../../flutter/LICENSE
@@ -13787,8 +13787,8 @@ FILE: ../../../flutter/shell/platform/linux/fl_plugin_registrar_test.cc
 FILE: ../../../flutter/shell/platform/linux/fl_plugin_registry.cc
 FILE: ../../../flutter/shell/platform/linux/fl_renderer.cc
 FILE: ../../../flutter/shell/platform/linux/fl_renderer.h
-FILE: ../../../flutter/shell/platform/linux/fl_renderer_gl.cc
-FILE: ../../../flutter/shell/platform/linux/fl_renderer_gl.h
+FILE: ../../../flutter/shell/platform/linux/fl_renderer_gdk.cc
+FILE: ../../../flutter/shell/platform/linux/fl_renderer_gdk.h
 FILE: ../../../flutter/shell/platform/linux/fl_renderer_headless.cc
 FILE: ../../../flutter/shell/platform/linux/fl_renderer_headless.h
 FILE: ../../../flutter/shell/platform/linux/fl_scrolling_manager.cc

shell/platform/linux/BUILD.gn

@@ -106,7 +106,6 @@ source_set("flutter_linux_sources") {
     "fl_dart_project.cc",
     "fl_engine.cc",
     "fl_event_channel.cc",
-    "fl_gl_area.cc",
     "fl_gnome_settings.cc",
     "fl_json_message_codec.cc",
     "fl_json_method_codec.cc",
@@ -127,7 +126,7 @@ source_set("flutter_linux_sources") {
     "fl_plugin_registrar.cc",
     "fl_plugin_registry.cc",
     "fl_renderer.cc",
-    "fl_renderer_gl.cc",
+    "fl_renderer_gdk.cc",
     "fl_renderer_headless.cc",
     "fl_scrolling_manager.cc",
     "fl_scrolling_view_delegate.cc",
@@ -247,7 +246,10 @@ executable("flutter_linux_unittests") {
 
   public_configs = [ "//flutter:config" ]
 
-  configs += [ "//flutter/shell/platform/linux/config:gtk" ]
+  configs += [
+    "//flutter/shell/platform/linux/config:gtk",
+    "//flutter/shell/platform/linux/config:epoxy",
+  ]
 
   defines = [
     "FLUTTER_ENGINE_NO_PROTOTYPES",

shell/platform/linux/fl_engine.cc

@@ -217,22 +217,14 @@ static void* fl_engine_gl_proc_resolver(void* user_data, const char* name) {
 
 static bool fl_engine_gl_make_current(void* user_data) {
   FlEngine* self = static_cast<FlEngine*>(user_data);
-  g_autoptr(GError) error = nullptr;
-  gboolean result = fl_renderer_make_current(self->renderer, &error);
-  if (!result) {
-    g_warning("%s", error->message);
-  }
-  return result;
+  fl_renderer_make_current(self->renderer);
+  return true;
 }
 
 static bool fl_engine_gl_clear_current(void* user_data) {
   FlEngine* self = static_cast<FlEngine*>(user_data);
-  g_autoptr(GError) error = nullptr;
-  gboolean result = fl_renderer_clear_current(self->renderer, &error);
-  if (!result) {
-    g_warning("%s", error->message);
-  }
-  return result;
+  fl_renderer_clear_current(self->renderer);
+  return true;
 }
 
 static uint32_t fl_engine_gl_get_fbo(void* user_data) {
@@ -248,12 +240,8 @@ static bool fl_engine_gl_present(void* user_data) {
 
 static bool fl_engine_gl_make_resource_current(void* user_data) {
   FlEngine* self = static_cast<FlEngine*>(user_data);
-  g_autoptr(GError) error = nullptr;
-  gboolean result = fl_renderer_make_resource_current(self->renderer, &error);
-  if (!result) {
-    g_warning("%s", error->message);
-  }
-  return result;
+  fl_renderer_make_resource_current(self->renderer);
+  return true;
 }
 
 // Called by the engine to retrieve an external texture.