EGL/X11 backend for WindowThumbnail QQuickItem

Introduces a new optional dependency to EGL. If EGL is found the
WindowThumbnail gets compiled with EGL support. The EGL support is
unlike Qt's XCB plugin not mutual exclusive to the GLX backend, thus
it's possible to compile with EGL and GLX at the same time.
By that this implementation is prepared for the case that Qt supports
EGL or GLX through runtime selection.

In practice EGL support is only useful if Qt is compiled with GLESv2.
In that case the corebindingsplugin gets linked against Qt5::Gui_GLESv2
to get the dependency to GLES.

The implementation makes use of the EGL_KHR_image extension
(or EGL_KHR_image_base and EGL_KHR_Image_pixmap) and
the GL_OES_EGL_image extension to bind the X pixmap to an EGLImageKHR.

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