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/meson.build

@@ -1,5 +1,6 @@
 lite_sources = [
     'api/api.c',
+    'api/cp_replace.c',
     'api/renderer.c',
     'api/renderer_font.c',
     'api/system.c',