(PhVideoEngine) Make sure _deinterlace is initialized

On Windows, it was implicitely initialized to true, and this
was causing VideoTest::deinterlaceTest to fail.
(PhGraphicTexturedRect) Fix the colors at the edges of the texture

Since the change to bilinear scaling, the colors at the edge were
wrong. The scaling by default uses a `repeat' mode, which is only
appropriate when drawing a repeating pattern. So we use `clamp to
edge' as appropriate here.

Since GL_CLAMP_TO_EDGE is defined in OpenGL 1.2 and since Windows
gl.h only provides OpenGL 1.1, switch to the QtOpenGL header
instead. QtOpenGL takes care of defining the proper OpenGL
constants.
(PhGraphicTexturedRect) Explain the scaling filter.
Remove QT+=testlib in all tests projects that do not use it.

Also remove the unused #include <QTest> in ConsoleTest, and the
unused #include <QtTests> in SonyTest.