Add explicit support for remapping variables.

Will enable use of extensions like framebuffer fetch, last fragment
depth and so on.

spirv_cross.cpp

@@ -2000,3 +2000,13 @@ ExecutionModel Compiler::get_execution_model() const
 {
 	return execution.model;
 }
+
+void Compiler::set_remapped_variable_state(uint32_t id, bool remap_enable)
+{
+	get<SPIRVariable>(id).remapped_variable = remap_enable;
+}
+
+bool Compiler::get_remapped_variable_state(uint32_t id) const
+{
+	return get<SPIRVariable>(id).remapped_variable;
+}

spirv_cross.hpp

@@ -171,6 +171,12 @@ public:
 	// Query shader resources, use ids with reflection interface to modify or query binding points, etc.
 	ShaderResources get_shader_resources() const;
 
+	// Remapped variables are considered built-in variables and a backend will
+	// not emit a declaration for this variable.
+	// This is mostly useful for making use of builtins which are dependent on extensions.
+	void set_remapped_variable_state(uint32_t id, bool remap_enable);
+	bool get_remapped_variable_state(uint32_t id) const;
+
 	// Query and modify OpExecutionMode.
 	uint64_t get_execution_mode_mask() const;
 	void unset_execution_mode(spv::ExecutionMode mode);

spirv_glsl.cpp

@@ -204,6 +204,9 @@ void CompilerGLSL::emit_header()
 {
 	statement("#version ", options.version, options.es && options.version > 100 ? " es" : "");
 
+	for (auto &header : header_lines)
+		statement(header);
+
 	// Needed for binding = # on UBOs, etc.
 	if (!options.es && options.version < 420)
 	{
@@ -4087,6 +4090,11 @@ void CompilerGLSL::add_resource_name(uint32_t id)
 	add_variable(resource_names, id);
 }
 
+void CompilerGLSL::add_header_line(const std::string &line)
+{
+	header_lines.push_back(line);
+}
+
 void CompilerGLSL::require_extension(const string &ext)
 {
 	if (forced_extensions.find(ext) == end(forced_extensions))

spirv_glsl.hpp

@@ -114,6 +114,22 @@ public:
 	}
 	std::string compile() override;
 
+	// Adds a line to be added right after #version in GLSL backend.
+	// This is useful for enabling custom extensions which are outside the scope of SPIRV-Cross.
+	// This can be combined with variable remapping.
+	// A new-line will be added.
+	//
+	// While add_header_line() is a more generic way of adding arbitrary text to the header
+	// of a GLSL file, require_extension() should be used when adding extensions since it will
+	// avoid creating collisions with SPIRV-Cross generated extensions.
+	//
+	// Code added via add_header_line() is typically backend-specific.
+	void add_header_line(const std::string &str);
+
+	// Adds an extension which is required to run this shader, e.g.
+	// require_extension("GL_KHR_my_extension");
+	void require_extension(const std::string &ext);
+
 protected:
 	void reset();
 	void emit_function(SPIRFunction &func, uint64_t return_flags);
@@ -288,7 +304,6 @@ protected:
 	// Can modify flags to remote readonly/writeonly if image type
 	// and force recompile.
 	bool check_atomic_image(uint32_t id);
-	void require_extension(const std::string &ext);
 
 	void replace_fragment_output(SPIRVariable &var);
 	void replace_fragment_outputs();
@@ -306,6 +321,7 @@ protected:
 	void track_expression_read(uint32_t id);
 
 	std::unordered_set<std::string> forced_extensions;
+	std::vector<std::string> header_lines;
 
 	uint32_t statement_count;