Add Missing check for buffer size divisible by scale

Fixes: #367

wl_surface.attach specification says:

"The new size of the surface is calculated based on the buffer size transformed by the inverse buffer_transform and the inverse buffer_scale. This means that the supplied buffer must be an integer multiple of the buffer_scale."

Since the specification is worded "must", this implements the check in weston and raise a protocol error if the requirement is violated. Note, that if wp_viewport is used to explicitly set the destination size, then the buffer size requirement is lifted.

Also adds test to check this error is raised. when specification is violated.