Add swapInterval to QSurfaceFormat

Implement swap interval support for EGL, GLX and WGL.

The environment variable QT_QPA_EGLFS_SWAPINTERVAL is renamed to
QT_QPA_EGL_SWAPINTERVAL and can be used to override the applications'
setting of the swap interval.

Task-number: QTBUG-31939

Change-Id: I644325d5d3306b7604bffd7efccda3c00ed37d36
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
Make drawing to and from RGBA8888 images faster

After profiling drawing with RGBA8888 images most of the time appears
to be spend in converting to and from ARGB32PM.

This patch adds four small converters that are 3-4 times faster than the
generic converter.

Change-Id: I3c7498756f440ca3ea9c1417b26dd8e1953b9d06
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>