fix fonts not working after a call to getContext

issue was limited to osx and windows

before this, if you create a canvas and get its context, the fonts would
freeze - you cannot add any fonts after getContext is called once, even
if you set up a new canvas. i fixed this by refreshing the Pango FontMap
(the component that does font lookups)

after you create a canvas+context, that canvas+context will not be able
to use new fonts, but that is less of a problem

src/register_font.cc

@@ -1,3 +1,7 @@
+#include <pango/pangocairo.h>
+#include <pango/pango-fontmap.h>
+#include <pango/pango.h>
+
 #ifdef __APPLE__
 #include <CoreFoundation/CoreFoundation.h>
 #include <CoreText/CoreText.h>
@@ -9,13 +13,22 @@
 
 bool
 register_font(unsigned char *filepath) {
+  bool success;
+
   #ifdef __APPLE__
   CFURLRef filepathUrl = CFURLCreateFromFileSystemRepresentation(NULL, filepath, strlen((char*)filepath), false);
-  return CTFontManagerRegisterFontsForURL(filepathUrl, kCTFontManagerScopeProcess, NULL);
+  success = CTFontManagerRegisterFontsForURL(filepathUrl, kCTFontManagerScopeProcess, NULL);
   #elif defined(_WIN32)
-  return AddFontResourceEx((LPCSTR)filepath, FR_PRIVATE, 0) != 0;
+  success = AddFontResourceEx((LPCSTR)filepath, FR_PRIVATE, 0) != 0;
   #else
-  return FcConfigAppFontAddFile(FcConfigGetCurrent(), (FcChar8 *)(filepath));
+  success = FcConfigAppFontAddFile(FcConfigGetCurrent(), (FcChar8 *)(filepath));
   #endif
+
+  // Tell Pango to throw away the current FontMap and create a new one. This
+  // has the effect of registering the new font in Pango by re-looking up all
+  // font families.
+  if (success) pango_cairo_font_map_set_default(NULL);
+
+  return success;
 }