Allow renderType: Text.NativeRendering on retina displays/devices

Setting the renderType to Text.NativeRendering might be needed in some
cases where distance-field does not produce the expected results, such
as for emoji characters or really large fonts. These use cases are
valid on retina displays as well, so having the setter second-guess
the request from the user to use native rendering is not ideal.

Change-Id: I7c6049766e60574487c29de07fbd5c100ec69a2a
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Move logic for determining native vs. DF text into single factory function

Makes QSGContext::createGlyphNode() the central point of determining which
glyph node to produce, instead of letting the caller call two different
versions of the factory, each one calling the other in various cases and
behind various ifdefs.

Change-Id: I30fb17cceab45d9e13ddf3ece7a65f220c5e5acd
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>