Load winsys and gl dynamically in the windows plugin

The dynamic builds (-opengl dynamic) are now functional on Windows.

In such a build no components in Qt link to any OpenGL libraries directly
and qmake will not automatically add any such libraries to the
applications' makefiles. Instead, the libraries are chosen and loaded
during runtime and applications are expected to use QOpenGLFunctions
instead of direct OpenGLfunction calls.

Set the environment variable QT_OPENGL to desktop or angle to skip testing
and force the given implementation. The application attributes (AA_UseOpenGLES
and such) are also taken into account.

The testing logic is same as before: We try to load opengl32 and
resolve a shader related function. If this fails, ANGLE is chosen. This
allows utilizing full desktop OpenGL on systems that have proper drivers,
while a transparent fallback to ANGLE will be done automatically for
systems that don't. The latter includes also remote desktop connections.

Software rendering via Mesa llvmpipe is supported too. The fallback is
automatic on systems where the desktop test fails and ANGLE fails to load
or initialize (e.g. due to missing libs like d3dcompiler), as long as a
suitable patched build of Mesa is available.

[ChangeLog][QtGui] Dynamic OpenGL implementation loading is now supported
on Windows. This requires Qt to be configured with -opengl dynamic.

Task-number: QTBUG-36483
Change-Id: Ie8bb25a6d55b3a1609b00150aeccd909aec27313
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>