st: Drop usage of gdk_cairo_set_source_pixbuf()

Avoid using it by... guess it... yes! Copying the function
into an utility. This helper function to convert between
pixel formats is quite self-contained and unrelated to the
rest of GTK, so may be copied in place.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2655>

src/st/st-image-content.c

@@ -21,6 +21,8 @@
 #include "st-image-content.h"
 #include "st-private.h"
 
+#include <gdk-pixbuf/gdk-pixbuf.h>
+
 struct _StImageContent
 {
   /*< private >*/

src/st/st-texture-cache.c

@@ -541,6 +541,121 @@ pixbuf_to_st_content_image (GdkPixbuf *pixbuf,
   return image;
 }
 
+static void
+util_cairo_surface_paint_pixbuf (cairo_surface_t *surface,
+                                 const GdkPixbuf *pixbuf)
+{
+  int width, height;
+  guchar *gdk_pixels, *cairo_pixels;
+  int gdk_rowstride, cairo_stride;
+  int n_channels;
+  int j;
+
+  if (cairo_surface_status (surface) != CAIRO_STATUS_SUCCESS)
+    return;
+
+  /* This function can't just copy any pixbuf to any surface, be
+   * sure to read the invariants here before calling it */
+  g_assert (cairo_surface_get_type (surface) == CAIRO_SURFACE_TYPE_IMAGE);
+  g_assert (cairo_image_surface_get_format (surface) == CAIRO_FORMAT_RGB24 ||
+            cairo_image_surface_get_format (surface) == CAIRO_FORMAT_ARGB32);
+  g_assert (cairo_image_surface_get_width (surface) == gdk_pixbuf_get_width (pixbuf));
+  g_assert (cairo_image_surface_get_height (surface) == gdk_pixbuf_get_height (pixbuf));
+
+  cairo_surface_flush (surface);
+
+  width = gdk_pixbuf_get_width (pixbuf);
+  height = gdk_pixbuf_get_height (pixbuf);
+  gdk_pixels = gdk_pixbuf_get_pixels (pixbuf);
+  gdk_rowstride = gdk_pixbuf_get_rowstride (pixbuf);
+  n_channels = gdk_pixbuf_get_n_channels (pixbuf);
+  cairo_stride = cairo_image_surface_get_stride (surface);
+  cairo_pixels = cairo_image_surface_get_data (surface);
+
+  for (j = height; j; j--)
+    {
+      guchar *p = gdk_pixels;
+      guchar *q = cairo_pixels;
+
+      if (n_channels == 3)
+        {
+          guchar *end = p + 3 * width;
+
+          while (p < end)
+            {
+#if G_BYTE_ORDER == G_LITTLE_ENDIAN
+              q[0] = p[2];
+              q[1] = p[1];
+              q[2] = p[0];
+#else
+              q[1] = p[0];
+              q[2] = p[1];
+              q[3] = p[2];
+#endif
+              p += 3;
+              q += 4;
+            }
+        }
+      else
+        {
+          guchar *end = p + 4 * width;
+          guint t1,t2,t3;
+
+#define MULT(d,c,a,t) G_STMT_START { t = c * a + 0x80; d = ((t >> 8) + t) >> 8; } G_STMT_END
+
+          while (p < end)
+            {
+#if G_BYTE_ORDER == G_LITTLE_ENDIAN
+              MULT(q[0], p[2], p[3], t1);
+              MULT(q[1], p[1], p[3], t2);
+              MULT(q[2], p[0], p[3], t3);
+              q[3] = p[3];
+#else
+              q[0] = p[3];
+              MULT(q[1], p[0], p[3], t1);
+              MULT(q[2], p[1], p[3], t2);
+              MULT(q[3], p[2], p[3], t3);
+#endif
+
+              p += 4;
+              q += 4;
+            }
+
+#undef MULT
+        }
+
+      gdk_pixels += gdk_rowstride;
+      cairo_pixels += cairo_stride;
+    }
+
+  cairo_surface_mark_dirty (surface);
+}
+
+static void
+util_cairo_set_source_pixbuf (cairo_t         *cr,
+                              const GdkPixbuf *pixbuf,
+                              gdouble          pixbuf_x,
+                              gdouble          pixbuf_y)
+{
+  cairo_format_t format;
+  cairo_surface_t *surface;
+
+  if (gdk_pixbuf_get_n_channels (pixbuf) == 3)
+    format = CAIRO_FORMAT_RGB24;
+  else
+    format = CAIRO_FORMAT_ARGB32;
+
+  surface = cairo_surface_create_similar_image (cairo_get_target (cr),
+                                                format,
+                                                gdk_pixbuf_get_width (pixbuf),
+                                                gdk_pixbuf_get_height (pixbuf));
+
+  util_cairo_surface_paint_pixbuf (surface, pixbuf);
+
+  cairo_set_source_surface (cr, surface, pixbuf_x, pixbuf_y);
+  cairo_surface_destroy (surface);
+}
+
 static cairo_surface_t *
 pixbuf_to_cairo_surface (GdkPixbuf *pixbuf)
 {
@@ -552,7 +667,7 @@ pixbuf_to_cairo_surface (GdkPixbuf *pixbuf)
   dummy_surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, 1, 1);
 
   cr = cairo_create (dummy_surface);
-  gdk_cairo_set_source_pixbuf (cr, pixbuf, 0, 0);
+  util_cairo_set_source_pixbuf (cr, pixbuf, 0, 0);
   pattern = cairo_get_source (cr);
   cairo_pattern_get_surface (pattern, &surface);
   cairo_surface_reference (surface);