Implement unicode character replacements

Useful to draw whitespaces with alternate characters and colors
without slowing down the text rendering.

A new API is implemented. A renderer.replacements object can be created
to list the replacements.

In turns the function renderer.draw_text and draw_text_subpixel now accept
two optional arguments for replacements.

src/api/api.h

@@ -6,6 +6,7 @@
 #include "lualib.h"
 
 #define API_TYPE_FONT "Font"
+#define API_TYPE_REPLACE "Replace"
 
 void api_load_libs(lua_State *L);