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/cp_replace.c

@@ -0,0 +1,42 @@
+#include "api.h"
+#include "renderer.h"
+
+
+static int f_new(lua_State *L) {
+  CPReplaceTable *rep_table = lua_newuserdata(L, sizeof(CPReplaceTable));
+  luaL_setmetatable(L, API_TYPE_REPLACE);
+  ren_cp_replace_init(rep_table);
+  return 1;
+}
+
+
+static int f_gc(lua_State *L) {
+  CPReplaceTable *rep_table = luaL_checkudata(L, 1, API_TYPE_REPLACE);
+  ren_cp_replace_free(rep_table);
+  return 0;
+}
+
+
+static int f_add(lua_State *L) {
+  CPReplaceTable *rep_table = luaL_checkudata(L, 1, API_TYPE_REPLACE);
+  const char *src = luaL_checkstring(L, 2);
+  const char *dst = luaL_checkstring(L, 3);
+  ren_cp_replace_add(rep_table, src, dst);
+  return 0;
+}
+
+
+static const luaL_Reg lib[] = {
+  { "__gc",               f_gc     },
+  { "new",                f_new    },
+  { "add",                f_add    },
+  { NULL, NULL }
+};
+
+int luaopen_renderer_replacements(lua_State *L) {
+  luaL_newmetatable(L, API_TYPE_REPLACE);
+  luaL_setfuncs(L, lib, 0);
+  lua_pushvalue(L, -1);
+  lua_setfield(L, -2, "__index");
+  return 1;
+}