net-libs/webkit-gtk: simplify opengl vs gles2 handling, fix gst gles2 deps

net-libs/webkit-gtk: simplify opengl vs gles2 handling, fix gst gles2 deps

webkit-gtk itself does not support both full OpenGL and GLESv2 at once yet.
ENABLE_OPENGL enables OpenGL in general, and if enabled then ENABLE_GLES2
determines which API is used - if enabled, then gles2, otherwise full GL.
Simplify REQUIRED_USE handling to enforce the opengl/gles2 conflict
separately, and have the webgl case just want either, to be future-proof
and easier to read independently.

WebGL should just always be enabled when either GL API is enabled, but
keeping it under USE flag for the time being, until an even deeper
GL/GStreamer webkitgtk cmake review, due to the webgl? ( gstreamer )
REQUIRED_USE still present.
For now just (probably) properly depend on gstreamer gles2 support for
USE="gles2 gstreamer" builds.

Bug: https://bugs.gentoo.org/500150
Package-Manager: Portage-2.3.43, Repoman-2.3.10