Change use of `float` to `CGFloat`.
Change `ceilf` usage to `ceil` instead.
Change drawingRectForBounds to have its Y ceiled when dH > 0.

Change-Id: I172f71456196defd3dfbc081be6c1e2b0b578a86
Merge pull request #55 from rastersize/fix-implicit-floating-point-precision-loss

Fix implicit floating point precision loss