xkbcommon should not be a mandatory dependency

Not all platforms that use qtwayland depend on xkbcommon to be present
for them to be functional

Change-Id: I9dcc8f661f7d6035f405420d9d4733b52c77112e
Reviewed-by: Andy Nichols <andy.nichols@digia.com>
Fix shader precision qualifier

On some stricter platforms this will cause problems in compiling
the shader code

Change-Id: I466e2ed231be9d414c9a3ffdb15ff67186933162
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
remove QWindowSystemInterface::flushWindowSystemEvents()

Change-Id: I88e83f4490f98752e827c743405b7f62e1910902
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>
Use shared .pri file for wayland_common and GL integrations

wayland_common.pro missed pkg-config for xkbcommon.
But wayland-common.pri has it. Actually, Both the .pro and the .pri
has many identical configurations. Make it to share
wayland_common_share.pri for those configuations including xkbcommon
to build wayland_common and GL integrations.

Change-Id: I397592eae482976c18d3e86bd8e0427a5f219d06
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>