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/cover.vert

@@ -21,7 +21,7 @@ layout(location = 1) in vec2 uv;
 layout(location = 1) out vec2 vUV;
 
 void main() {
-    gl_Position = vec4(pos*transform.xy + transform.zw, z, 1);
+    gl_Position = toClipSpace(vec4(pos*transform.xy + transform.zw, z, 1));
 	vUV = uv*uvTransform.xy + uvTransform.zw;
 	vec3[2] fboTrans = fboTextureTransform();
 	vec3 uv3 = transform3x2(fboTrans, vec3(uv, 1.0));