[pango] attributes: Make alpha work without color

[Matthias Clasen <matthiasc src gnome org>]

commit 97fc6fbffe65d9ca4e92fc8979808faebed0ee35
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Apr 7 22:49:00 2017 -0400

    attributes: Make alpha work without color
    
    Improve the behavior of the cairo renderer to take
    alpha into account even if no color specified.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=773767

 pango/pangocairo-render.c |   37 +++++++++++++++++++++++++++++--------
 1 files changed, 29 insertions(+), 8 deletions(-)
---
diff --git a/pango/pangocairo-render.c b/pango/pangocairo-render.c
index b1efb37..73b1ae3 100644
--- a/pango/pangocairo-render.c
+++ b/pango/pangocairo-render.c
@@ -191,17 +191,38 @@ set_color (PangoCairoRenderer *crenderer,
           PangoRenderPart     part)
 {
   PangoColor *color = pango_renderer_get_color ((PangoRenderer *) (crenderer), part);
-  guint16 alpha = pango_renderer_get_alpha ((PangoRenderer *) (crenderer), part);
+  guint16 a = pango_renderer_get_alpha ((PangoRenderer *) (crenderer), part);
+  gdouble red, green, blue, alpha;
 
-  if (!alpha)
-    alpha = 0xffff;
+  if (!a && !color)
+    return;
 
   if (color)
-    cairo_set_source_rgba (crenderer->cr,
-                          color->red / 65535.,
-                          color->green / 65535.,
-                          color->blue / 65535.,
-                           alpha / 65535.);
+    {
+      red = color->red / 65535.;
+      green = color->green / 65535.;
+      blue = color->blue / 65535.;
+      alpha = 1.;
+    }
+  else
+    {
+      cairo_pattern_t *pattern = cairo_get_source (crenderer->cr);
+
+      if (pattern && cairo_pattern_get_type (pattern) == CAIRO_PATTERN_TYPE_SOLID)
+        cairo_pattern_get_rgba (pattern, &red, &green, &blue, &alpha);
+      else
+        {
+          red = 0.;
+          green = 0.;
+          blue = 0.;
+          alpha = 1.;
+        }
+    }
+
+  if (a)
+    alpha = a / 65535.;
+
+  cairo_set_source_rgba (crenderer->cr, red, green, blue, alpha);
 }
 
 /* note: modifies crenderer->cr without doing cairo_save/restore() */