Fix insane compile errors with Haxe 3.1.2

Haxe 3.1.2 is too stupid to recognize "GradientType"
is not flash.display.GradientType in GradientFill.hx...

Also suddenly, SpreadMethod.normal is not allowed,
which has been for years.

I conclude that Haxe is still developed by a bunch
of amateurs and we can no longer support Haxe 3.0.
It is simply impossible to differentiate between
version 3.1.2 and 3.0 in Haxe code.

So much for semantic versioning...