gsk: Linearize colors too

Convert all the GdkRGBA colors that we have
in render nodes to linear sRGB before using
them. This brings the rendering back in line
with what cairo produces, except for the video.

gsk/gl/gskglrenderjob.c

@@ -27,6 +27,7 @@
 #include <gdk/gdkprofilerprivate.h>
 #include <gdk/gdkrgbaprivate.h>
 #include <gdk/gdkcolorspaceprivate.h>
+#include <gdk/gdkmemoryformatprivate.h>
 #include <gsk/gskrendernodeprivate.h>
 #include <gsk/gskglshaderprivate.h>
 #include <gdk/gdktextureprivate.h>
@@ -989,11 +990,23 @@ gsk_gl_render_job_update_clip (GskGLRenderJob        *job,
   return TRUE;
 }
 
+/* Convert from sRGB floats to linear sRGB half-floats */
 static inline void
 rgba_to_half (const GdkRGBA *rgba,
               guint16        h[4])
 {
-  float_to_half4 ((const float *)rgba, h);
+  GdkRGBA d;
+  gdk_memory_convert ((guchar *) &d,
+                      sizeof (float) * 4,
+                      GDK_MEMORY_R32G32B32A32_FLOAT,
+                      gdk_color_space_get_srgb_linear (),
+                      (guchar *) rgba,
+                      sizeof (float) * 4,
+                      GDK_MEMORY_R32G32B32A32_FLOAT,
+                      gdk_color_space_get_srgb (),
+                      1,
+                      1);
+  float_to_half4 ((const float *)&d, h);
 }
 
 /* fill_vertex_data */