Do not rely on scene measurements during retexture; the DSMultilineLabelNode might not yet have been added to an SKScene.
Default paragraph width using scene if it exists, or UIScreen/NSScreen width otherwise.
When calculating bounding box for text, use MAXFLOAT for height so that it is unrestricted.
Keep anchorPoint the same when retexturing: This allows the owner to set a custom anchorPoint and not have the value reset to (0.5,0.5) on other property changes.
Suppress floating point precision warnings: In 32-bit build, ceil() returns a double, which must be cast back to a CGFloat (float). In 64-bit build, the cast is unnecessary, since CGFloat is a double, but it is also harmless.
Fix crash in retexture when self.text is nil or empty.
See https://github.com/downrightsimple/DSMultilineLabelNode/issues/4
When text breaks exactly at the paragraph width, the bounding box calculation code will decide to wrap it, reserving vertical space for another line, but the drawing code won't, leaving the vertical space empty.  It's hard to know what the correct solution is; this fixes a test case, but it might break others.