Update to shader generator constructors

Author: niklasharrysson

source/MaterialXGenGlsl/EsslShaderGenerator.h

@@ -21,10 +21,7 @@ class MX_GENGLSL_API EsslShaderGenerator : public GlslShaderGenerator
 {
   public:
     /// Constructor.
-    /// Optionally pass in an externally created TypeSystem here, 
-    /// if you want to keep type descriptions alive after the lifetime
-    /// of the shader generator.
-    EsslShaderGenerator(TypeSystemPtr typeSystem = TypeSystem::create());
+    EsslShaderGenerator(TypeSystemPtr typeSystem);
 
     /// Creator function.
     /// Optionally pass in an externally created TypeSystem here, 

source/MaterialXGenGlsl/GlslShaderGenerator.h

@@ -23,10 +23,7 @@ class MX_GENGLSL_API GlslShaderGenerator : public HwShaderGenerator
 {
   public:
     /// Constructor.
-    /// Optionally pass in an externally created TypeSystem here, 
-    /// if you want to keep type descriptions alive after the lifetime
-    /// of the shader generator.
-    GlslShaderGenerator(TypeSystemPtr typeSystem = TypeSystem::create());
+    GlslShaderGenerator(TypeSystemPtr typeSystem);
 
     /// Creator function.
     /// Optionally pass in an externally created TypeSystem here, 

source/MaterialXGenMdl/MdlShaderGenerator.h

@@ -54,10 +54,7 @@ class MX_GENMDL_API MdlShaderGenerator : public ShaderGenerator
 {
   public:
     /// Constructor.
-    /// Optionally pass in an externally created TypeSystem here, 
-    /// if you want to keep type descriptions alive after the lifetime
-    /// of the shader generator.
-    MdlShaderGenerator(TypeSystemPtr typeSystem = TypeSystem::create());
+    MdlShaderGenerator(TypeSystemPtr typeSystem);
 
     /// Creator function.
     /// Optionally pass in an externally created TypeSystem here, 

source/MaterialXGenMsl/MslShaderGenerator.h

@@ -26,10 +26,7 @@ class MX_GENMSL_API MslShaderGenerator : public HwShaderGenerator
 {
   public:
     /// Constructor.
-    /// Optionally pass in an externally created TypeSystem here, 
-    /// if you want to keep type descriptions alive after the lifetime
-    /// of the shader generator.
-    MslShaderGenerator(TypeSystemPtr typeSystem = TypeSystem::create());
+    MslShaderGenerator(TypeSystemPtr typeSystem);
 
     /// Creator function.
     /// Optionally pass in an externally created TypeSystem here, 

source/MaterialXGenOsl/OslShaderGenerator.h

@@ -24,10 +24,7 @@ class MX_GENOSL_API OslShaderGenerator : public ShaderGenerator
 {
   public:
     /// Constructor.
-    /// Optionally pass in an externally created TypeSystem here, 
-    /// if you want to keep type descriptions alive after the lifetime
-    /// of the shader generator.
-    OslShaderGenerator(TypeSystemPtr typeSystem = TypeSystem::create());
+    OslShaderGenerator(TypeSystemPtr typeSystem);
 
     /// Creator function.
     /// Optionally pass in an externally created TypeSystem here,