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>

compositor/compositor.c

@@ -664,6 +664,7 @@ fade_output(struct wlsc_output *output,
 	surface.height = output->current->height;
 	surface.texture = GL_NONE;
 	surface.transform = NULL;
+	surface.alpha = compositor->current_alpha;
 
 	if (tint <= 1.0)
 		surface.visual = WLSC_PREMUL_ARGB_VISUAL;