VideoCommon/ShaderGenCommon: Add function for writing fmt-based strings

Begins the conversion of the shader generators over to using fmt
formatting specifiers.

This also has a benefit over the older StringFromFormat-based API in
that all formatted data is appended to the existing buffer rather than
creating a completely separate string and then appending it to the
internal string buffer.

Source/Core/VideoCommon/ShaderGenCommon.h

@@ -6,11 +6,14 @@
 
 #include <cstdarg>
 #include <cstring>
+#include <iterator>
 #include <map>
 #include <string>
 #include <type_traits>
 #include <vector>
 
+#include <fmt/format.h>
+
 #include "Common/CommonTypes.h"
 #include "Common/StringUtil.h"
 #include "VideoCommon/VideoCommon.h"
@@ -101,6 +104,8 @@ class ShaderCode : public ShaderGeneratorInterface
 public:
   ShaderCode() { m_buffer.reserve(16384); }
   const std::string& GetBuffer() const { return m_buffer; }
+
+  // Deprecated: Writes format strings using traditional printf format strings.
   void Write(const char* fmt, ...)
 #ifdef __GNUC__
       __attribute__((format(printf, 2, 3)))
@@ -112,6 +117,13 @@ public:
     va_end(arglist);
   }
 
+  // Writes format strings using fmtlib format strings.
+  template <typename... Args>
+  void WriteFmt(std::string_view format, Args&&... args)
+  {
+    fmt::format_to(std::back_inserter(m_buffer), format, std::forward<Args>(args)...);
+  }
+
 protected:
   std::string m_buffer;
 };