Rewrite host GPU abstraction

- Don't have to repeat the same thing for 4 renderers.
 - Add native Metal renderer.

src/util/shadergen.h

@@ -3,7 +3,7 @@
 
 #pragma once
 
-#include "host_display.h"
+#include "gpu_device.h"
 
 #include <sstream>
 #include <string>
@@ -20,10 +20,15 @@ public:
   std::string GenerateUVQuadVertexShader();
   std::string GenerateFillFragmentShader();
   std::string GenerateCopyFragmentShader();
-  std::string GenerateSampleFragmentShader();
+  std::string GenerateDisplayVertexShader();
+  std::string GenerateDisplayFragmentShader(bool set_alpha_to_one = false);
+
+  std::string GenerateImGuiVertexShader();
+  std::string GenerateImGuiFragmentShader();
 
 protected:
   ALWAYS_INLINE bool IsVulkan() const { return (m_render_api == RenderAPI::Vulkan); }
+  ALWAYS_INLINE bool IsMetal() const { return (m_render_api == RenderAPI::Metal); }
 
   const char* GetInterpolationQualifier(bool interface_block, bool centroid_interpolation, bool sample_interpolation,
                                         bool is_out) const;
@@ -52,9 +57,11 @@ protected:
 
   RenderAPI m_render_api;
   bool m_glsl;
+  bool m_spirv;
   bool m_supports_dual_source_blend;
   bool m_use_glsl_interface_blocks;
   bool m_use_glsl_binding_layout;
+  bool m_has_uniform_buffer = false;
 
   std::string m_glsl_version_string;
 };