allow renaming of shader entry point when creating SPIR-V

Use "--source-entrypoint name" on the command line, or the
TShader::setSourceEntryPoint(char*) API.

When the name given to the above interfaces is detected in the
shader source, it will be renamed to the entry point name supplied
to the -e option or the TShader::setEntryPoint() method.

Test/hlsl.entry.rename.frag

@@ -0,0 +1,15 @@
+
+struct PS_OUTPUT
+{
+    float4 Color : SV_Target0;
+};
+
+void not_the_entry_point() { }
+int also_not_the_entry_point;
+
+PS_OUTPUT main()
+{
+    PS_OUTPUT psout;
+    psout.Color = 0;
+    return psout;
+}