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;
 }

gpu/shaders/common.inc

@@ -12,6 +12,12 @@ vec3[2] fboTextureTransform() {
 	return t;
 }
 
+// toClipSpace converts an OpenGL gl_Position value to a
+// native GPU position.
+vec4 toClipSpace(vec4 pos) {
+	return pos;
+}
+
 vec3 transform3x2(vec3[2] t, vec3 v) {
 	return vec3(dot(t[0], v), dot(t[1], v), dot(vec3(0.0, 0.0, 1.0), v));
 }

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));