gst-libs/gst/gl/egl/egl.h collides with EGL/egl.h on case-insensitive platforms

Regression from !784 (merged)

This breaks the UWP gstgl build since it uses EGL (ANGLE). Will break EGL on macOS and iOS too, but we don't build that afaik, so we didn't notice.

We need to rename egl.h to something else.

I think maybe we'll want to rename all of the top-level headers, not just egl.h.