Don't increment the paint's generation ID when drawing bitmaps

When the renderer draws a bitmap as part of a display list with an
alpha < 1.0f, the paint is temporarily modified to alter the opacity
of the bitmap. This has the side effect of increasing the paint's
generation ID counter which can break paint caching.

Change-Id: I5055d059ad1639829fa50af3d946e296c4dab877