Hook Steam screenshots and send internal capture

Using the Steamworks API, we can hook the screenshot function and listen
for a screenshot request callback to send in our own screenshot. This
applies the screenshot improvements to Steam screenshots as well.

Doing this requires adding some C wrapper functions, as our interface
with the Steam API is only conducted through C.

desktop_version/src/CWrappers.cpp

@@ -2,6 +2,8 @@
 
 #include <SDL.h>
 
+#include "Graphics.h"
+#include "GraphicsUtil.h"
 #include "Localization.h"
 #include "UtilityClass.h"
 
@@ -27,4 +29,14 @@ uint32_t LOC_toupper_ch(uint32_t ch)
     return loc::toupper_ch(ch);
 }
 
+SDL_Surface* GRAPHICS_tempScreenshot(void)
+{
+    return graphics.tempScreenshot;
+}
+
+uint8_t UTIL_TakeScreenshot(SDL_Surface** surface)
+{
+    return TakeScreenshot(surface);
+}
+
 } /* extern "C" */