Depth buffer images

[Mark Janes]

Hello José,

I'm adding support for visualizing depth/stencil buffers to
frameretrace, and found that the png support in apitrace converts
grayscale images to 8 bits per pixel.

For the applications I'm looking at, the depth buffer values are mostly
near 1.0f, and the resulting image has visible banding due to the low
precision.

I'd like to support 32bit grayscale images, but I am not very familiar
with libpng.  I'll hack at image_png.cpp, but please let me know if
there is something else I should be considering.

-Mark