Update the calculation of bitmap image based on more complicated rules for most use cases

[dreampiggy]

Extend from #28

Rules here:

For example, given a SVG image which original size is (40,50):

preserve aspect ratio	width	height	effect
true	50	50	(40,50)
true	0	100	(80,100)
true	20	0	(20,25)
true	0	0	(40,50)
false	50	50	(50,50)
false	0	100	(40,100)
false	20	0	(20,50)
false	0	0	(40,50)