gl_video: support packed YUV formats with Apple extensions

This adds support for packed YUV formats (YUVY and UYVY) using the extension
GL_APPLE_rgb_422. While supporting this formats on their own is not that
important (considering most video is planar YUV) they are used for
interoperability with IOSurfaces.

Next commit will use this formats to render VDA hardware decoded frames through
IOSurface and OpenGL interoperability.
vo_opengl: support for vda hardware decoding

The harder work was done in the previous commits. After that this feature comes
out almost for free.

The only problem is I can't get the textures created with CGLTexImageIOSurface2D
to download properly, thus the code performs download using some CoreVideo APIs.

If someone knows why download of textures created with CGLTexImageIOSurface2D
doesn't work please contact me :)
options: add option to disable using right Alt key as Alt Gr

mpv was hardcoded to always consider the right Alt key as Alt Gr, but there
are parituclar combinations of platforms and keyboard layouts where it's more
convenient to treat the right Alt as a keyboard modifier just like the left
one.

Fixes #388