Fix memory leak in MTFont

[maitbayev]

MTFont doesn't handle memory management correctly for CTFontRef and CGFontRef. The ownership policy for Core Foundation as following:

Foundation:

• If you create an object (either directly or by making a copy of another object—see The Create Rule), you own it.
• If you get an object from somewhere else, you do not own it. If you want to prevent it being disposed of, you must add yourself as an owner (using CFRetain).
• If you are an owner of an object, you must relinquish ownership when you have finished using it (using CFRelease).

The Create Rule:

Core Foundation functions have names that indicate when you own a returned object:

• Object-creation functions that have “Create” embedded in the name
• Object-duplication functions that have “Copy” embedded in the name

Per the above policy, MTFont shouldn't call CFRetain after CTFontCreateWithGraphicsFont or CGFontCreateWithDataProvider since CFRetain is already invoked when creating the objects.

[maitbayev]

@kostub PTAL