gpu/shaders: introduce toClipSpace to map to GPU native clip space

OpenGL use the [-1; 1] range for clip depths, Direct3D [0; 1].
Use toClipSpace to encapsulate the difference.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/shaders/blit.vert

@@ -4,6 +4,8 @@
 
 precision highp float;
 
+#include <common.inc>
+
 layout(binding = 0) uniform Block {
 	vec4 transform;
 	vec4 uvTransform;
@@ -18,6 +20,6 @@ layout(location = 0) out vec2 vUV;
 
 void main() {
 	vec2 p = pos*transform.xy + transform.zw;
-	gl_Position = vec4(p, z, 1);
+	gl_Position = toClipSpace(vec4(p, z, 1));
 	vUV = uv*uvTransform.xy + uvTransform.zw;
 }