package/opencv: add opengl support

In OpenCV, OpenGL is only used by highgui module.

OpenGL support is done using extensions from 3rd party framework: either
Qt5OpenGL with Qt5 (with GL support only, not GLES); or gtkglext (which
is not available in Buildroot) with gtk2

So, make OpenGL knob a sub-option of the Qt5 support option.

Note: we enclose both the GUI toolkit choice and the GL option in an
if-block, so that the GL option gets properly indented; having it
depend on WITH_QT5 is not enough, because it does not directly follow
it, so kconfig would not consider it for indenting.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
[yann.morin.1998@free.fr: tweak commit log about the if-block]
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>