Do not use flexible member arrays in C++.

This is supported as compiler extensions, but we can be standards
compliant here, so use the classic workaround of having a single entry
array at the end of the structs instead.

spirv_cpp.cpp

@@ -251,6 +251,7 @@ string CompilerCPP::compile()
 	backend.basic_uint_type = "uint32_t";
 	backend.swizzle_is_function = true;
 	backend.shared_is_implied = true;
+	backend.flexible_member_array_supported = false;
 
 	uint32_t pass_count = 0;
 	do

spirv_glsl.cpp

@@ -3735,7 +3735,19 @@ string CompilerGLSL::type_to_array_glsl(const SPIRType &type)
 	{
 		res += "[";
 		if (size)
+		{
 			res += convert_to_string(size);
+		}
+		else if (!backend.flexible_member_array_supported)
+		{
+			// For runtime-sized arrays, we can work around
+			// lack of standard support for this by simply having
+			// a single element array.
+			//
+			// Runtime length arrays must always be the last element
+			// in an interface block.
+			res += '1';
+		}
 		res += "]";
 	}
 	return res;

spirv_glsl.hpp

@@ -203,6 +203,7 @@ protected:
 		const char *basic_uint_type = "uint";
 		bool swizzle_is_function = false;
 		bool shared_is_implied = false;
+		bool flexible_member_array_supported = true;
 	} backend;
 
 	void emit_struct(const SPIRType &type);