Use oiiowriter plugin to writer DPX files

[cchampet]

:warning: 3 parameters of the dpxwriter are not ported to the oiiowriter:

• transfer: metadata that defines the amplitude transfer function used to transform the data from a linear original.
  If needed, there is an oiio property: dpx:Transfer.
• colorimetric: metadata that defines the appropriate color reference primaries (for additive color systems like television) or color responses (for printing density).
  If needed, there is an oiio property: dpx:Colorimetric.
• packed: the component data packing method.
  If needed, there is an oiio property: dpx:Packing.

[cchampet]

I think you could also remove the PNG reader/writer plugins. These plugins have almost the same performance to decode/encode png images (5~10% better than oiio), but the oiio writer plugin has a lot of features the png writer doesn't have (subsampling, metadata...). @fabiencastan What do you think?