Fixes from review of PR #190.

spirv_cross.cpp

@@ -1469,6 +1469,10 @@ void Compiler::parse(const Instruction &instruction)
 		type.image.sampled = ops[6];
 		type.image.format = static_cast<ImageFormat>(ops[7]);
 		type.image.access = (length >= 9) ? static_cast<AccessQualifier>(ops[8]) : AccessQualifierMax;
+
+		if (type.image.sampled == 0)
+			SPIRV_CROSS_THROW("OpTypeImage Sampled parameter must not be zero.");
+
 		break;
 	}
 

spirv_glsl.cpp

@@ -2893,11 +2893,6 @@ void CompilerGLSL::emit_texture_op(const Instruction &i)
 	auto &type = expression_type(img);
 	auto &imgtype = get<SPIRType>(type.self);
 
-	// Mark that this shader reads from this image
-	auto *p_var = maybe_get_backing_variable(img);
-	if (p_var)
-		unset_decoration(p_var->self, DecorationNonReadable);
-
 	uint32_t coord_components = 0;
 	switch (imgtype.image.dim)
 	{

spirv_msl.cpp

@@ -1179,10 +1179,18 @@ void CompilerMSL::emit_instruction(const Instruction &instruction)
 
 	// Images
 
-	// Reads == fetches in Metal
+	// Reads == Fetches in Metal
 	case OpImageRead:
+	{
+		// Mark that this shader reads from this image
+		uint32_t img_id = ops[2];
+		auto *p_var = maybe_get_backing_variable(img_id);
+		if (p_var)
+			unset_decoration(p_var->self, DecorationNonReadable);
+
 		emit_texture_op(instruction);
 		break;
+	}
 
 	case OpImageWrite:
 	{
@@ -2490,7 +2498,7 @@ string CompilerMSL::image_type_glsl(const SPIRType &type, uint32_t id)
 	// For unsampled images, append the sample/read/write access qualifier.
 	// For kernel images, the access qualifier my be supplied directly by SPIR-V.
 	// Otherwise it may be set based on whether the image is read from or written to within the shader.
-	if (type.basetype == SPIRType::Image)
+	if (type.basetype == SPIRType::Image && type.image.sampled == 2)
 	{
 		switch (img_type.access)
 		{