[gtk/wip/otte/gleanup: 70/73] gl: Fix issue with EGL + nvidia

[Chun-wei Fan <fanchunwei src gnome org>]

commit bd8b4da57a0b62788c583b2ef6775c49e88368da
Author: Benjamin Otte <otte redhat com>
Date:   Sat Jul 10 02:29:17 2021 +0200

    gl: Fix issue with EGL + nvidia
    
    nvidia sets the default draw buffer to GL_NONE if EGL contexts are
    initially bound to EGL_NO_SURFACE which is exactly what we are doing. So
    bind them to GL_BACK when drawing, as they should be.
    
    See https://phabricator.services.mozilla.com/D118743 for a discussion
    about EGL_NO_CONTEXT and draw buffers.

 gdk/wayland/gdkglcontext-wayland.c | 11 +++++++++++
 gdk/x11/gdkglcontext-egl.c         | 10 ++++++++++
 2 files changed, 21 insertions(+)
---
diff --git a/gdk/wayland/gdkglcontext-wayland.c b/gdk/wayland/gdkglcontext-wayland.c
index 558c9cfed0..3e1fbe3f59 100644
--- a/gdk/wayland/gdkglcontext-wayland.c
+++ b/gdk/wayland/gdkglcontext-wayland.c
@@ -276,6 +276,16 @@ gdk_wayland_gl_context_make_current (GdkGLContext *context,
                          egl_surface,
                          context_wayland->egl_context);
 }
+
+static void
+gdk_wayland_gl_context_begin_frame (GdkDrawContext *draw_context,
+                                    cairo_region_t *region)
+{
+  GDK_DRAW_CONTEXT_CLASS (gdk_wayland_gl_context_parent_class)->begin_frame (draw_context, region);
+
+  glDrawBuffer (GL_BACK);
+}
+
 static void
 gdk_wayland_gl_context_end_frame (GdkDrawContext *draw_context,
                                   cairo_region_t *painted)
@@ -337,6 +347,7 @@ gdk_wayland_gl_context_class_init (GdkWaylandGLContextClass *klass)
 
   gobject_class->dispose = gdk_wayland_gl_context_dispose;
 
+  draw_context_class->begin_frame = gdk_wayland_gl_context_begin_frame;
   draw_context_class->end_frame = gdk_wayland_gl_context_end_frame;
 
   context_class->realize = gdk_wayland_gl_context_realize;
diff --git a/gdk/x11/gdkglcontext-egl.c b/gdk/x11/gdkglcontext-egl.c
index b5a571303c..40c458b2d4 100644
--- a/gdk/x11/gdkglcontext-egl.c
+++ b/gdk/x11/gdkglcontext-egl.c
@@ -310,6 +310,15 @@ gdk_x11_surface_destroy_egl_surface (GdkX11Surface *self)
   self->egl_surface = NULL;
 }
 
+static void
+gdk_x11_gl_context_egl_begin_frame (GdkDrawContext *draw_context,
+                                    cairo_region_t *region)
+{
+  GDK_DRAW_CONTEXT_CLASS (gdk_x11_gl_context_egl_parent_class)->begin_frame (draw_context, region);
+
+  glDrawBuffer (GL_BACK);
+}
+
 static void
 gdk_x11_gl_context_egl_end_frame (GdkDrawContext *draw_context,
                                   cairo_region_t *painted)
@@ -637,6 +646,7 @@ gdk_x11_gl_context_egl_class_init (GdkX11GLContextEGLClass *klass)
   context_class->clear_current = gdk_x11_gl_context_egl_clear_current;
   context_class->get_damage = gdk_x11_gl_context_egl_get_damage;
 
+  draw_context_class->begin_frame = gdk_x11_gl_context_egl_begin_frame;
   draw_context_class->end_frame = gdk_x11_gl_context_egl_end_frame;
 
   gobject_class->dispose = gdk_x11_gl_context_egl_dispose;