Color fonts scale incorrectly after caching

Steps to reproduce:

1. Render a piece of text at pixel size 12 containing some emoji characters using a color font (e.g. Emoji One)

2. Render a second piece of text at pixel size 48, containing emoji characters not used in the previous line

3. Render a third piece of text containing more unique emoji at pixel size 12

Expected result:
Line 1 should render at 12px, line 2 at 48px and line 3 at 12px.

Actual result:
Line 1 renderers at 12px and line 2 at 48px, however line 3 also renders at 48px instead of 12px.

This happens as a result of the scalableBitmapFactor being shared between QFontEngineFT instances and only updated when an engine is initialised for the first time. Because a 12px version of the engine has already been cached from rendering line 1 it doesn't get reinitialised for line 3 and so keeps using the scalableBitmapFactor from line 2 (48px).

The attached patch associates the scalableBitmapFactor with the QFontEngineFT so that it's preserved when restoring from the cache.