sw_engine: revise scale transform logic.

sw_engine: revise scale transform logic.

Basically, stroke width size is linear,

engine couldn't apply scale factor from the matrix which contains 2 dimensional values.

Thus, we can apply it if the scale factor of x/y is identical.

Otherwise, we should transform every stroke points in the stroking process.
That scenario can be improved with another patch.

Change-Id: I070dcf29d2e42f21e182bdf4239781464158ef73