screen-share: Avoid bit-shifting large values

Found while running with b_sanitize=undefined, which yields:

runtime error: shift exponent 909199186 is too large for 32-bit type 'int'

Converts the shm_formats to a boolean and checks for the correct pixel
format it directly, instead of trying to gather them all in an array and
then later on to do the check.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>