API: Add gdk_display_create_gl_context()

This is an alternative to gdk_surface_create_gl_context() when the
context is meant to only draw to textures.

This is useful in the testsuite or in GStreamer or with GLArea,
basically whenever we want to do GL stuff but don't need to actually
draw anything on screen.

A bunch of code will need to be updated to deal with context->surface
being NULL.

gdk/gdkdisplay.c

@@ -1325,6 +1325,38 @@ gdk_display_prepare_gl (GdkDisplay  *self,
     }
 }
 
+/**
+ * gdk_display_create_gl_context:
+ * @self: a `GdkDisplay`
+ * @error: return location for an error
+ *
+ * Creates a new `GdkGLContext` for the `GdkDisplay`.
+ *
+ * The context is disconnected from any particular surface or surface
+ * and cannot be used to draw to any surface. It can only be used to
+ * draw to non-surface framebuffers like textures.
+ *
+ * If the creation of the `GdkGLContext` failed, @error will be set.
+ * Before using the returned `GdkGLContext`, you will need to
+ * call [method@Gdk.GLContext.make_current] or [method@Gdk.GLContext.realize].
+ *
+ * Returns: (transfer full) (nullable): the newly created `GdkGLContext`
+ *
+ * Since: 4.6
+ */
+GdkGLContext *
+gdk_display_create_gl_context (GdkDisplay  *self,
+                               GError     **error)
+{
+  g_return_val_if_fail (GDK_IS_DISPLAY (self), NULL);
+  g_return_val_if_fail (error == NULL || *error == NULL, NULL);
+
+  if (!gdk_display_prepare_gl (self, error))
+    return NULL;
+
+  return gdk_gl_context_new (self, NULL);
+}
+
 /*< private >
  * gdk_display_get_gl_context:
  * @self: the `GdkDisplay`

gdk/gdkdisplay.h

@@ -71,6 +71,9 @@ gboolean    gdk_display_supports_input_shapes (GdkDisplay    *display);
 GDK_AVAILABLE_IN_4_4
 gboolean    gdk_display_prepare_gl         (GdkDisplay  *self,
                                             GError     **error);
+GDK_AVAILABLE_IN_4_6
+GdkGLContext *gdk_display_create_gl_context(GdkDisplay  *self,
+                                            GError     **error);
 
 GDK_AVAILABLE_IN_ALL
 GdkDisplay *gdk_display_get_default (void);

gdk/gdkglcontext.c

@@ -722,15 +722,19 @@ gdk_gl_context_init (GdkGLContext *self)
 
 /* Must have called gdk_display_prepare_gl() before */
 GdkGLContext *
-gdk_gl_context_new_for_surface (GdkSurface *surface)
+gdk_gl_context_new (GdkDisplay *display,
+                    GdkSurface *surface)
 {
-  GdkDisplay *display = gdk_surface_get_display (surface);
-  GdkGLContext *shared = gdk_display_get_gl_context (display);
+  GdkGLContext *shared;
+
+  g_assert (surface == NULL || display == gdk_surface_get_display (surface));
 
   /* assert gdk_display_prepare_gl() had been called */
+  shared = gdk_display_get_gl_context (display);
   g_assert (shared);
 
   return g_object_new (G_OBJECT_TYPE (shared),
+                       "display", display,
                        "surface", surface,
                        NULL);
 }

gdk/gdkglcontextprivate.h

@@ -99,7 +99,8 @@ gboolean                gdk_gl_backend_can_be_used              (GdkGLBackend
                                                                  GError         **error);
 void                    gdk_gl_backend_use                      (GdkGLBackend     backend_type);
 
-GdkGLContext *          gdk_gl_context_new_for_surface          (GdkSurface      *surface);
+GdkGLContext *          gdk_gl_context_new                      (GdkDisplay      *display,
+                                                                 GdkSurface      *surface);
 
 gboolean                gdk_gl_context_is_api_allowed           (GdkGLContext    *self,
                                                                  GdkGLAPI         api,

gdk/gdksurface.c

@@ -1183,7 +1183,7 @@ gdk_surface_create_gl_context (GdkSurface   *surface,
   if (!gdk_display_prepare_gl (surface->display, error))
     return NULL;
 
-  return gdk_gl_context_new_for_surface (surface);
+  return gdk_gl_context_new (surface->display, surface);
 }
 
 /**