Implement taking smallmap screenshots.

This is largely based on this patch:
http://www.tt-forums.net/viewtopic.php?p=849533#p849533
with the following changes:
* this uses a button in the smallmap window instead of a console command
* screenshot now uses zoom level and mode of smallmap window

src/screenshot.h

@@ -26,8 +26,11 @@ enum ScreenshotType {
 	SC_HEIGHTMAP,   ///< Heightmap of the world.
 };
 
+class SmallMapWindow;
+
 void SetupScreenshotViewport(ScreenshotType t, struct ViewPort *vp);
 bool MakeHeightmapScreenshot(const char *filename);
+bool MakeSmallMapScreenshot(unsigned int width, unsigned int height, SmallMapWindow *window);
 bool MakeScreenshot(ScreenshotType t, const char *name);
 
 extern char _screenshot_format_name[8];