[client] egl: generate header file with #define for desktop shader

Instead of duplicating the #defines from the shader into the C code,
this commit adds a custom CMake rule that exports all the #defines
from a shader into a C header.

client/renderers/EGL/desktop.c

@@ -33,9 +33,7 @@ Place, Suite 330, Boston, MA 02111-1307 USA
 // these headers are auto generated by cmake
 #include "desktop.vert.h"
 #include "desktop_rgb.frag.h"
-
-#define LG_SCALE_NEAREST 0
-#define LG_SCALE_LINEAR  1
+#include "desktop_rgh.def.h"
 
 struct DesktopShader
 {