gl_shader_decompiler: Add AddLine() overload that forwards to fmt

In a lot of places throughout the decompiler, string concatenation via
operator+ is used quite heavily. This is usually fine, when not heavily
used, but when used extensively, can be a problem. operator+ creates an
entirely new heap allocated temporary string and given we perform
expressions like:

std::string thing = a + b + c + d;

this ends up with a lot of unnecessary temporary strings being created
and discarded, which kind of thrashes the heap more than we need to.
Given we utilize fmt in some AddLine calls, we can make this a part of
the ShaderWriter's API. We can make an overload that simply acts as a
passthrough to fmt.

This way, whenever things need to be appended to a string, the operation
can be done via a single string formatting operation instead of
discarding numerous temporary strings. This also has the benefit of
making the strings themselves look nicer and makes it easier to spot
errors in them.

src/video_core/renderer_opengl/gl_shader_decompiler.cpp

@@ -69,6 +69,17 @@ public:
         AddNewLine();
     }
 
+    // Overload the forwards all arguments directly to libfmt.
+    // Note that all formatting requirements for fmt must be
+    // obeyed when using this function. (e.g. {{ must be used
+    // printing the character '{' is desirable. Ditto for }} and '}',
+    // etc).
+    template <typename... Args>
+    void AddLine(std::string_view text, Args&&... args) {
+        AddExpression(fmt::format(text, std::forward<Args>(args)...));
+        AddNewLine();
+    }
+
     void AddNewLine() {
         DEBUG_ASSERT(scope >= 0);
         shader_source += '\n';