VideoBackends:Metal: Cache pipelines

Metal pipelines hold less stuff than Dolphin pipelines, so duplicates will appear

Source/Core/VideoBackends/Metal/MTLShader.mm

@@ -0,0 +1,19 @@
+
+#include "VideoBackends/Metal/MTLShader.h"
+
+#include "VideoBackends/Metal/MTLObjectCache.h"
+
+Metal::Shader::Shader(ShaderStage stage, std::string msl, MRCOwned<id<MTLFunction>> shader)
+    : AbstractShader(stage), m_msl(std::move(msl)), m_shader(std::move(shader))
+{
+}
+
+Metal::Shader::~Shader()
+{
+  g_object_cache->ShaderDestroyed(this);
+}
+
+AbstractShader::BinaryData Metal::Shader::GetBinary() const
+{
+  return BinaryData(m_msl.begin(), m_msl.end());
+}