Generate a PNG from a raster of RGBA integer values.
Use this operation when you have created a raster whose values are already
RGBA color values that you wish to render into a PNG. If you have a raster
with data that you wish to render, you should use RenderPng instead.
An RGBA value is a 32 bit integer with 8 bits used for each component:
the first 8 bits are the red value (between 0 and 255), then green, blue,
and alpha (with 0 being transparent and 255 being opaque).