gpu: fix clip intersection with the D3D backend

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

gpu/shaders/common.inc

@@ -17,6 +17,23 @@ vec3[2] fboTextureTransform() {
 	return t;
 }
 
+// fboTransform returns a transformation
+// that cancels the implied transformation between
+// the clip space and the framebuffer.
+// Only two rows are returned. The last is implied
+// to be [0, 0, 1].
+vec3[2] fboTransform() {
+	vec3[2] t;
+#ifdef HLSL
+	t[0] = vec3(1.0, 0.0, 0.0);
+	t[1] = vec3(0.0, 1.0, 0.0);
+#else
+	t[0] = vec3(1.0, 0.0, 0.0);
+	t[1] = vec3(0.0, -1.0, 0.0);
+#endif
+	return t;
+}
+
 // toClipSpace converts an OpenGL gl_Position value to a
 // native GPU position.
 vec4 toClipSpace(vec4 pos) {

gpu/shaders/intersect.vert

@@ -4,18 +4,25 @@
 
 precision highp float;
 
+#include <common.inc>
+
 layout(location = 0) in vec2 pos;
 layout(location = 1) in vec2 uv;
 
 layout(binding = 0) uniform Block {
 	vec4 uvTransform;
+	vec4 subUVTransform;
 };
 
 layout(location = 0) out vec2 vUV;
 
 void main() {
-  vec2 p = pos;
-  p.y = -p.y;
-  gl_Position = vec4(p, 0, 1);
-  vUV = uv*uvTransform.xy + uvTransform.zw;
+  vec3[2] fboTrans = fboTransform();
+  vec3 p = transform3x2(fboTrans, vec3(pos, 1.0));
+  gl_Position = vec4(p, 1);
+  vec3[2] fboTexTrans = fboTextureTransform();
+  vec3 uv3 = transform3x2(fboTexTrans, vec3(uv, 1.0));
+  vUV = uv3.xy*subUVTransform.xy + subUVTransform.zw;
+  vUV = transform3x2(fboTexTrans, vec3(vUV, 1.0)).xy;
+  vUV = vUV*uvTransform.xy + uvTransform.zw;
 }