TextField::setTextColor: fix textColor not updating after setting it

This fixes https://github.com/openfl/openfl-native/issues/41

I realize that this is probably an inrecidbly hacky / inefficient fix, but it works... and I couldn't get the "obvious solution" (or at least what seemed obvious to me) to work, that is, setting one of the different dirty flags:

```cpp
mLinesDirty = true;
mFontsDirty = true;
mGfxDirty = true;

dirtyCache();
```