gl: Use GL_BACK_LEFT

The spec doesn't allow GL_BACK, but many drivers do. Use the
spec-accepted equivalent instead.

Fixed #4402

diff --git a/gsk/gl/gskglrenderjob.c b/gsk/gl/gskglrenderjob.c
index a904cf09d48021d189081391fa955600d95dca2c..1862191118763f25b2c15853b9c56a9a71ddd671 100644 (file)
--- a/gsk/gl/gskglrenderjob.c
+++ b/gsk/gl/gskglrenderjob.c
@@ -4073,7 +4073,7 @@ get_framebuffer_format (guint framebuffer)
   glBindFramebuffer (GL_FRAMEBUFFER, framebuffer);
   glGetFramebufferAttachmentParameteriv (GL_FRAMEBUFFER,
                                          framebuffer ? GL_COLOR_ATTACHMENT0
-                                                     : GL_BACK,
+                                                     : GL_BACK_LEFT,
                                          GL_FRAMEBUFFER_ATTACHMENT_RED_SIZE, &size);
 
   if (size > 16)