GLSL: fix clip space mismatch in gl_Position.

OpenGL clips against [-w, w], while WebGPU uses [0, w]. The fix is to
modify gl_Position.z on output to match GLSL semantics. (This is the
same code used by SPIRV-Cross under the fixup_clipspace option.)

Bug: tint:1401
Change-Id: I1b1511352eee11cd9b095cd809aa8e1263c6cf4c
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/78261
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>

test/samples/compute_boids.wgsl.expected.glsl

@@ -56,6 +56,7 @@ void main() {
   tint_symbol_3 outputs;
   outputs = vert_main(inputs);
   gl_Position = outputs.value;
+  gl_Position.z = 2.0 * gl_Position.z - gl_Position.w;
   gl_Position.y = -gl_Position.y;
 }
 

test/samples/cube.wgsl.expected.glsl

@@ -59,6 +59,7 @@ void main() {
   outputs = vtx_main(inputs);
   vtxFragColor = outputs.vtxFragColor;
   gl_Position = outputs.Position;
+  gl_Position.z = 2.0 * gl_Position.z - gl_Position.w;
   gl_Position.y = -gl_Position.y;
 }
 

test/samples/simple_vertex.spvasm.expected.glsl

@@ -31,6 +31,7 @@ void main() {
   tint_symbol_2 outputs;
   outputs = tint_symbol_1();
   gl_Position = outputs.tint_symbol;
+  gl_Position.z = 2.0 * gl_Position.z - gl_Position.w;
   gl_Position.y = -gl_Position.y;
 }
 

test/samples/triangle.wgsl.expected.glsl

@@ -30,6 +30,7 @@ void main() {
   tint_symbol_2 outputs;
   outputs = vtx_main(inputs);
   gl_Position = outputs.value;
+  gl_Position.z = 2.0 * gl_Position.z - gl_Position.w;
   gl_Position.y = -gl_Position.y;
 }