Set Metal semantics for SkSL point size.

Metal requires any variable in the vertex shader representing point size
to be assigned the [[point_size]] semantic. Otherwise, point size is
undefined.

Bug: skia:10658
Change-Id: Iac428175327a27ba319e9dab9a0c17d4b1731cce
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/314318
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

src/sksl/SkSLMetalCodeGenerator.cpp

@@ -1501,7 +1501,7 @@ void MetalCodeGenerator::writeOutputStruct() {
         }
     }
     if (fProgram.fKind == Program::kVertex_Kind) {
-        this->write("    float sk_PointSize;\n");
+        this->write("    float sk_PointSize [[point_size]];\n");
     }
     this->write("};\n");
 }