gl: Only enabled cocoa backend on osx (and not ios)

ios requires using EAGL
gl: fix crash if _build_extension_string is not called

On GLES2 then (gl->GetIntegerv && gl->GetStringi) is false
regression introduced by cc6df204e2f58fffda5cbe90f3450aeba95889c4

https://bugzilla.gnome.org/show_bug.cgi?id=703343
pkgconfig: add gstreamer-gl

https://bugzilla.gnome.org/show_bug.cgi?id=703343
gl: remove commented and unsued code in x11 Makefile.am

https://bugzilla.gnome.org/show_bug.cgi?id=703343
gl: deploy egl headers in gst/gl/egl instead of gst/gl

https://bugzilla.gnome.org/show_bug.cgi?id=703343
gl: add EGLImage support

* picked from old libgstegl:
  - GstEGLImageMemory
  - GstEGLImageAllocator
  - last_buffer management from removed GstEGLImageBufferPool

* add-ons:
  - GstEGLImageMemory now old a reference on GstGLContext
    so that it can delete the EGLImage and its gltexture source
    while having the associated gl context being current.
  - add EGLImage support for GstVideoGLTextureUploadMeta which
    mainly call EGLImageTargetTexture2D
  - GstGLBufferPool now supports GstEGLImageAllocator
  - glimagesink / glfilters / etc.. now propose GstEGLImageAllocator
    to upstream

https://bugzilla.gnome.org/show_bug.cgi?id=703343
gl: let the user includes itself our egl headers if needed

Forgot to address this change.

https://bugzilla.gnome.org/show_bug.cgi?id=703343
configure.ac: fix OSX host check

Makes the check works with (i386|x86_64)-apple-darwin* hosts.

https://bugzilla.gnome.org/show_bug.cgi?id=727104
gl: only set CAPS_FEATURE_MEMORY_EGL_IMAGE on egl platform
gl: fix out-of-source builds pt2

21b897de2f762bee504cbc007fd108148ee5ae24 did this for x11 and wayland.
Do it for the other backends as well.
gl: don't fail configure if we do not understand the platform
eglglessink: fix wrong doc block for create_window

Wrong since 1ad253b9 when prop name was changed from
can_create_window to create_window.
eglglessink: unref last buffer on DRAIN

Similar to 093574053fae243ec5fcfd6adae8185053b1e896
in gstbasesink

https://bugzilla.gnome.org/show_bug.cgi?id=726107
gl/win32: allow not building the gl plugins

Fixes build on windows if <GL/wglext.h> headers are not present.

https://bugzilla.gnome.org/show_bug.cgi?id=727125
tests: fix build of gl unit tests in uninstalled setup

Put local .la files first, and add link to libgstvideo
for tests that use the libgstvideo API directly.
tests: don't link codecparser tests twice to the lib
tests: fix LDADD order for insertbin test
build: Fix make dist
gl: egl: fix distcheck and out of source build
hlsdemux: Implement parsing of #EXT-X-I-FRAME-STREAM-INF

These are I-frame-only variant lists that can be used
for trick mode playback.