The existing conversions weren't handling gamma correctly and used
an ad-hoc definition of gray instead of based on true luminance.
[ChangeLog][QtGui] RGB conversions to grayscale formats are now
gamma-corrected and produce color-space luminance values
Change-Id: I88ab870c8f5e502ddb053e6a14a75102239a26f2
Reviewed-by: Lars Knoll <lars.knoll@qt.io>