backend-drm: Set default plane color space and range hints

Make the KMS backend and the gl-renderer explicitly ask for the same default values in order to produce more consistent results and rely less on driver specific default values.

This is also preparation for landing the color representation protocol (see Draft: color: Dummy implementations for color r... (!1514)).

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See also:

• https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4204

libweston/backend-drm/drm-kms-enums.h

@@ -93,7 +93,7 @@ enum wdrm_plane_color_encoding {
 	WDRM_PLANE_COLOR_ENCODING_BT2020,
 	WDRM_PLANE_COLOR_ENCODING__COUNT
 };
-#define WDRM_PLANE_COLOR_ENCODING_DEFAULT WDRM_PLANE_COLOR_ENCODING_BT601
+#define WDRM_PLANE_COLOR_ENCODING_DEFAULT WDRM_PLANE_COLOR_ENCODING_BT709
 
 /**
  * Possible values for the WDRM_PLANE_COLOR_RANGE property.