[[ Skia Update ]] Fix emscripen builds

[montegoulding]

This patch fixes the missing font manager factory for emscripten due to a
change in skia. This patch also improves the exclusions of mac and
windows files as they were being included in emscripten also.

[runrevmark]

@montegoulding It's not clear from the patch what the issue was exactly - are we including the appropraite FontManager to ensure that emscripten can still render text?

[montegoulding]

The main issue was that is src/ports/SkFontMgr_custom_directory_factory.cpp was being excluded. There is no src/ports/SkFontMgr_linux.cpp any more which is what was being used previously. The custom directory one loads fonts from a directory and because we were defining SK_FONT_FILE_PREFIX I presumed that's what we needed. The only other place that appears is in the android font manager.

[runrevmark]

Got it - thanks :) That at least suggests perhaps fixing the font issue on Android 7 might be done for us by Skia... If it now has a fallback implementation...

@livecode-vulcan review ok 3ec1df6

[livecode-vulcan]

💙 review by @runrevmark ok 3ec1df6