gstate: Skip ops with a clear mask.

As pointed out by Benjamin Otte, these are expensive no-ops that we can
trivially detect, just so long as we remember the semantics of extend
modes.

src/cairo-gstate.c

@@ -1019,6 +1019,12 @@ _cairo_gstate_mask (cairo_gstate_t  *gstate,
     if (_cairo_pattern_is_opaque (mask, NULL))
 	return _cairo_gstate_paint (gstate);
 
+    if (_cairo_pattern_is_clear (mask) &&
+	_cairo_operator_bounded_by_mask (gstate->op))
+    {
+	return CAIRO_STATUS_SUCCESS;
+    }
+
     _cairo_gstate_copy_transformed_source (gstate, &source_pattern.base);
     _cairo_gstate_copy_transformed_mask (gstate, &mask_pattern.base, mask);
 

src/cairo-pattern.c

@@ -1835,6 +1835,29 @@ _cairo_pattern_is_opaque (const cairo_pattern_t *abstract_pattern,
     return FALSE;
 }
 
+cairo_bool_t
+_cairo_pattern_is_clear (const cairo_pattern_t *abstract_pattern)
+{
+    const cairo_pattern_union_t *pattern;
+
+    if (abstract_pattern->has_component_alpha)
+	return FALSE;
+
+    pattern = (cairo_pattern_union_t *) abstract_pattern;
+    switch (pattern->type) {
+    case CAIRO_PATTERN_TYPE_SOLID:
+	return pattern->solid.color.alpha_short == 0x0000;
+    case CAIRO_PATTERN_TYPE_SURFACE:
+	return pattern->surface.surface->is_clear &&
+	       pattern->surface.surface->content & CAIRO_CONTENT_ALPHA;
+    default:
+	ASSERT_NOT_REACHED;
+    case CAIRO_PATTERN_TYPE_LINEAR:
+    case CAIRO_PATTERN_TYPE_RADIAL:
+	return FALSE;
+    }
+}
+
 /**
  * _cairo_pattern_analyze_filter:
  * @pattern: surface pattern

src/cairoint.h

@@ -2531,6 +2531,9 @@ cairo_private cairo_bool_t
 _cairo_pattern_is_opaque (const cairo_pattern_t *pattern,
 			  const cairo_rectangle_int_t *extents);
 
+cairo_private cairo_bool_t
+_cairo_pattern_is_clear (const cairo_pattern_t *pattern);
+
 enum {
     CAIRO_PATTERN_ACQUIRE_NONE = 0x0,
     CAIRO_PATTERN_ACQUIRE_NO_REFLECT = 0x1,