Adjust the values of dirty rectangles that have negative width or height.
When a DisplayNode object has negative scales, its dirty rectangle has negative width or height. Unfortunately, my renderer cannot create a clip rectangle that covers it for this case. This change changes the position (and size) of a dirty rectangle that has negative values to fix this problem.
Merge pull request #50 from tkihira/hbono/negative-rectangles

Adjust the values of a dirty rectangle that has negative values