MSL: Fix duplicate gl_Position outputs when gl_Position defined but unused.

When gl_Position is defined by SPIR-V, but neither used nor initialized,
it appeared twice in the MSL output, as gl_Position and glPosition_1.

The existing tests for whether an output is active check only that it is
used by an op, or initialized. Adding the implicit gl_Position also marked
the existing gl_Position as active, duplicating the output variable.

Fix is that when checking for the need to add an implicit gl_Position
output, also check if the var is already defined in the shader,
and just needs to be marked as active.
Add test shader.

1 files changed, 13 insertions, 0 deletions

diff --git a/shaders-msl/vert/unused-position.vert b/shaders-msl/vert/unused-position.vert
new file mode 100644
index 00000000..61e30b43
--- /dev/null
+++ b/shaders-msl/vert/unused-position.vert
@@ -0,0 +1,13 @@
+#version 450
+
+out gl_PerVertex
+{
+    vec4 gl_Position;
+    float gl_PointSize;
+    float gl_ClipDistance[1];
+};
+
+void main()
+{
+    gl_PointSize = 1.0;
+}