fixes around isOpaque and dithering

fixes around isOpaque and dithering

- copyTo() now preserves isOpaqueness, and BitmapCopyTest tests it
- bitmap shader doesn't claim to have shadespan16 if dithering is on, since its
  sampler doesn't auto-dither (note that gradients do auto-dither in their
  16bit sampler)
- blitter setup just relies on the shader to report if its 16bit sampler can be
  called (allowing gradients to say yes regardless of dither, but bitmaps to say
  no if dithering is on)

git-svn-id: http://skia.googlecode.com/svn/trunk@331 2bbb7eff-a529-9590-31e7-b0007b416f81