[QFontEngineFT] Use QScopedArrayPointer to auto-free the bitmap buffer

Also track the allocated buffer length and re-use the existing
buffer instead of allocating a smaller one, where possible.

Change-Id: I494519d653522622b1fe4ec80ac9035e2c3d9714
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>