vo_gl: add new vo name "gl_sdl" to make SDL+GL mode available

vo_gl: add new vo name "gl_sdl" to make SDL+GL mode available

The actual work is done by the existing SDL code. This commit merely
makes it possible to explicitly select the SDL backend ("gl" alone
uses SDL only if the X11 and win32 backends are not available, while
the new "gl_sdl" always forces use of SDL).

Also disable YUV conversion method autodetection when SDL is used.
This gets rid of a temporary window that appears for a moment and is
immediately closed again. SDL can't deal with the VOFLAG_HIDDEN flag,
which is needed to create an invisible GL context (when the
autodetection is run, the video size isn't yet known to the VO, and
creating a window then resizing would cause problems with window
placement). Instead always pick the fragment program method by default
(yuv=2). This change affects the normal "gl" VO too if it chooses the
SDL backend.