method changes for hidpi/retina renderer changes in 3.0

[benfry]

When using a retina/hidpi version of a PGraphics (or PImage, for that matter), all pixel operations happen in 2x space. This has an effect on:

• updatePixels(...)
• get(...)
• set(...)
• copy(...)
• blend(...)

The pixelFactor variable is 2 for retina, 1 normally. The pixelWidth and pixelHeight fields cover the actual size of the pixels[] array.

[REAS]

OK, I tried to clarify this, mostly with added description in pixelDensity(), but with new examples in pixelWidth and pixelHeight. Some additions to pixels[] as well. Let's see how this goes.