colorstate: Add gdk_color_state_get_rendering_color_state()

Returns the linear color state that renderers should render in when
this is the target color state.

We disable this function unless linear compositing is enabled and just
return @self by default.

gdk/gdkcolorstate.c

@@ -20,8 +20,6 @@
 
 #include "gdkcolorstateprivate.h"
 
-#include "gdkdebugprivate.h"
-
 /**
  * GdkColorState:
  *
@@ -165,6 +163,7 @@ GdkDefaultColorState gdk_default_color_states[] = {
       .klass = &GDK_DEFAULT_COLOR_STATE_CLASS,
       .ref_count = 0,
       .depth = GDK_MEMORY_U8,
+      .rendering_color_state = GDK_COLOR_STATE_SRGB_LINEAR,
     },
     .name = "srgb",
     .no_srgb = GDK_COLOR_STATE_SRGB_LINEAR,
@@ -174,6 +173,7 @@ GdkDefaultColorState gdk_default_color_states[] = {
       .klass = &GDK_DEFAULT_COLOR_STATE_CLASS,
       .ref_count = 0,
       .depth = GDK_MEMORY_U8,
+      .rendering_color_state = GDK_COLOR_STATE_SRGB_LINEAR,
     },
     .name = "srgb-linear",
     .no_srgb = NULL,

gdk/gdkcolorstateprivate.h

@@ -1,6 +1,8 @@
 #pragma once
 
 #include "gdkcolorstate.h"
+
+#include "gdkdebugprivate.h"
 #include "gdkmemoryformatprivate.h"
 
 typedef enum
@@ -19,6 +21,7 @@ struct _GdkColorState
   gatomicrefcount ref_count;
 
   GdkMemoryDepth depth;
+  GdkColorState *rendering_color_state;
 };
 
 struct _GdkColorStateClass
@@ -52,6 +55,15 @@ extern GdkDefaultColorState gdk_default_color_states[GDK_COLOR_STATE_N_IDS];
 const char *    gdk_color_state_get_name                (GdkColorState          *color_state);
 GdkColorState * gdk_color_state_get_no_srgb_tf          (GdkColorState          *self);
 
+static inline GdkColorState *
+gdk_color_state_get_rendering_color_state (GdkColorState *self)
+{
+  if (!GDK_DEBUG_CHECK (LINEAR))
+    return self;
+
+  return self->rendering_color_state;
+}
+
 static inline GdkMemoryDepth
 gdk_color_state_get_depth (GdkColorState *self)
 {