Fix girth of native text with grayscale AA or style

When using a grayscale glyph cache, the source data should be
interpreted differently, like in the OpenGL paint engine. We
should not do srgb gamma correction and we should use the default
blend mode. This also fixes styled text, since that will always
default to a grayscale glyph cache.

This patch ensures:
1. That we detect when the font requires grayscale AA (e.g. when
font smoothing is turned off in the OS)
2. That we interpret the source data correctly.

Task-number: QTBUG-31720
Change-Id: I600f4a28cd443caa0a8d5544d219b47436cde693
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>