opengl: split into convenience library

Create a libvlc_opengl and libvlc_opengles library that are built only
if one other target is needing it, avoiding to compile the OpenGL code
once per module using it and removing the need for OPENGL_COMMON* vars.

As the fact we're using OpenGL or OpenGL ES is defined at compile time,
the clients must use the correct variant depending on what they use.

In addition, this patch refactor the glesv2 detection in order to
enable both the gles2 display plugin and the libvlc_opengles.la target
which must not be built on Windows target for example.
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merge request