QtGui/math3d: Fix QQuaternion::getEulerAngles for GimbalLock cases

This is heavily inspired by the patch written by Inho Lee
<inho.lee@qt.io>, which says "There is a precision problem in the
previous algorithm when checking pitch value. (In the case that the
rotation on the X-axis makes Gimbal lock.)"

In order to work around the precision problem, this patch does:

 1. switch to the algorithm described in the inline comment to make
    the story simple.
 2. forcibly normalize the {x, y, z, w} components to eliminate
    fractional errors.
 3. set threshold to avoid hidden division by cos(pitch) =~ 0.

From my testing which compares dot product of the original quaternion
and the one recreated from Euler angles, calculation within float range
seems okay. (abs(normalize(q_orig) * normalize(q_roundtrip)) >= 0.99999)

Many thanks to Inho Lee for the original patch and discussion about
rounding errors.

Fixes: QTBUG-72103
Pick-to: 6.3 6.2 5.15
Change-Id: I8995e4affe603111ff2303a0dfcbdb0b1ae03f10
Reviewed-by: Yuya Nishihara <yuya@tcha.org>
Reviewed-by: Inho Lee <inho.lee@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

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