RESIZE: Clarify dimension limits

X and Y dimensions of image limited
to 16-bit for integer data types so that
the position * stride calculations do not
overflow an int32_t (both position and stride
are then 16-bit)

Signed-off-by: Dominic Symes <dominic.symes@arm.com>
Change-Id: I24d15b1f2991a18da15493bef478d5ee9c65dba3