Metal: Fix Z-fighting in EEVEE Next

Small change to always opt-in to using
invariant position in the vertex shader.
This ensures precision between position
calculations from different shaders which
need to produce the exact same result, by
disabling fastMath on only those instructions.

After benchmarking, the impact of this change
does not appear to affect performance bottlenecks
but will reduce the need for additional bias calculations.

Authored by Apple: Michael Parkin-White.

Pull Request: https://projects.blender.org/blender/blender/pulls/117478

source/blender/gpu/metal/mtl_shader.mm

@@ -314,6 +314,7 @@ bool MTLShader::finalize(const shader::ShaderCreateInfo *info)
     MTLCompileOptions *options = [[[MTLCompileOptions alloc] init] autorelease];
     options.languageVersion = MTLLanguageVersion2_2;
     options.fastMathEnabled = YES;
+    options.preserveInvariance = YES;
 
     if (@available(macOS 11.00, *)) {
       /* Raster order groups for tile data in struct require Metal 2.3.