Fix compilation errors on GLES platforms

* Do not include glx.h when using GLES

GL/glx.h is included on all LINUX plattforms, which is wrong
for a number of reasons:

- GL_PERSPECTIVE_CORRECTION_HINT is defined in GL/gl.h, so we
  want gl.h not glx.h, the latter just includes the former
- GL/gl.h is a Desktop GL header, and should not be included
  on GLES plattforms
- GL/gl.h is already included via QtOpenGL ->
  QtGui/qopengl.h on desktop plattforms

This fixes a problem when Qt is compiled with GLES, which
is often done on ARM platforms where desktop GL is not or
only poorly supported (e.g. slow due to emulation).

Fixes part of opencv#9171.

* Only set GL_PERSPECTIVE_CORRECTION_HINT when GL version defines it

GL_PERSPECTIVE_CORRECTION_HINT does not exist in GLES 2.0/3.x,
and has been deprecated in OpenGL 3.0 core profiles.

Fixes part of opencv#9171.

* added note about BORDER_TYPE in separable filters; fixed SGBMStereo description

* added # to BORDER_ constants to generate hyperlinks

* Update py_thresholding.markdown

* 📝 Update py_thresholding.markdown

…i_issue

* fixed issue with Mat reshape in decodeMulti()

* added test for decodeMulti

* used assign

* fixed antialiased line rendering to process image border correctly

* fixed warning on Windows

* imgproc(test): circle drawing regression