egl: turn one more boolean `int` into a `bool`

egl: turn one more boolean `int` into a `bool`

Same as the previous commit, but this one was split out because it's
a bit more complicated: this field is given as a pointer to a function,
so the function had to be changed as well, and the function was use in
a bunch of places, which needed updating as well.

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>