gsk/gl: only clear glyph cache durign reclaimation

We don't need to clear the front cache on every frame as we can clear it
specifically when we do reclaimation to avoid unnecessary memset() calls.

gsk/gl/gskglglyphlibrary.c

@@ -84,12 +84,12 @@ gsk_gl_glyph_value_free (gpointer data)
 }
 
 static void
-gsk_gl_glyph_library_begin_frame (GskGLTextureLibrary *library,
-                                  gint64               frame_id,
-                                  GPtrArray           *removed_atlases)
+gsk_gl_glyph_library_clear_cache (GskGLTextureLibrary *library)
 {
   GskGLGlyphLibrary *self = (GskGLGlyphLibrary *)library;
 
+  g_assert (GSK_IS_GL_GLYPH_LIBRARY (self));
+
   memset (self->front, 0, sizeof self->front);
 }
 
@@ -111,7 +111,7 @@ gsk_gl_glyph_library_class_init (GskGLGlyphLibraryClass *klass)
 
   object_class->finalize = gsk_gl_glyph_library_finalize;
 
-  library_class->begin_frame = gsk_gl_glyph_library_begin_frame;
+  library_class->clear_cache = gsk_gl_glyph_library_clear_cache;
 }
 
 static void

gsk/gl/gskgltexturelibrary.c

@@ -141,6 +141,7 @@ gsk_gl_texture_library_begin_frame (GskGLTextureLibrary *self,
                                     GPtrArray            *removed_atlases)
 {
   GHashTableIter iter;
+  gboolean drop_caches = FALSE;
 
   g_return_if_fail (GSK_IS_GL_TEXTURE_LIBRARY (self));
 
@@ -175,6 +176,8 @@ gsk_gl_texture_library_begin_frame (GskGLTextureLibrary *self,
                 if (dropped > 0)
                   g_message ("%s: Dropped %d items",
                              G_OBJECT_TYPE_NAME (self), dropped));
+
+      drop_caches |= dropped > 0;
     }
 
   if (frame_id % self->max_frame_age == 0)
@@ -208,7 +211,12 @@ gsk_gl_texture_library_begin_frame (GskGLTextureLibrary *self,
                                         g_hash_table_size (self->hash_table),
                                         atlased,
                                         g_hash_table_size (self->hash_table) - atlased));
+
+      drop_caches |= dropped > 0;
     }
+
+  if (drop_caches && GSK_GL_TEXTURE_LIBRARY_GET_CLASS (self)->clear_cache)
+    GSK_GL_TEXTURE_LIBRARY_GET_CLASS (self)->clear_cache (self);
 }
 
 static GskGLTexture *

gsk/gl/gskgltexturelibraryprivate.h

@@ -103,6 +103,7 @@ typedef struct _GskGLTextureLibraryClass
   void (*begin_frame) (GskGLTextureLibrary *library,
                        gint64               frame_id,
                        GPtrArray           *removed_atlases);
+  void (*clear_cache) (GskGLTextureLibrary *library);
 } GskGLTextureLibraryClass;
 
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (GskGLTextureLibrary, g_object_unref)