compositor: fix uniform handling for fade_output()

compositor: fix uniform handling for fade_output()

fade_output() is strange in that it manufactures a wlsc_surface object
by hand, and then calls wlsc_surface_draw() on it.

Valgrind complained, that wlsc_surface_draw() accesses uninitialised
data: wlsc_surface::alpha. fade_output() forgets to set it.

Initialise surface.alpha in fade_output(). Specifically, set it to
compositor->current_alpha to deliberatly avoid the gluniform1f() call in
wlsc_surface_draw().

fade_output() binds a different GL shader program than
wlsc_surface_draw() expects. This program does not have a uniform called
"alpha", and the uniform location given in glUniform1f() is not for
this program anyway. A hint of that is the runtime error:
Mesa: User error: GL_INVALID_OPERATION in glUniform(type mismatch)

Fixing this seems to get rid of half a thousand of Valgrind errors, and
of course the Mesa user error.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>