gpu/shaders: compensate for GPU Framebuffer => texture transformation

Add fboTextureTransform shader function for cancelling the
implied transformation from fragments output by the fragment
shader and the (u, v) coordinates used to sample from it in a
later pass.

For OpenGL the transformation is the identity.

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

gpu/shaders/common.inc

@@ -1,4 +1,17 @@
-#version 310 es
+// SPDX-License-Identifier: Unlicense OR MIT
 
-void blah() {
+// fboTextureTransform returns a transformation
+// that cancels the implied transformation between
+// the framebuffer and its texture.
+// Only two rows are returned. The last is implied
+// to be [0, 0, 1].
+vec3[2] fboTextureTransform() {
+	vec3[2] t;
+	t[0] = vec3(1.0, 0.0, 0.0);
+	t[1] = vec3(0.0, 1.0, 0.0);
+	return t;
+}
+
+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));
 }