null image input & output

[lgritz]

This I/O plugin for "null" images are a file "format" where output of a null file does nothing at all (no I/O, not even any data copying), and input of a null file just fills the result buffers with black pixels (0 values), with no I/O or data copying.

What's this good for?

• Benchmarking, if you want to have OIIO's input or output truly take as close to no time whatsoever.

• "Dry run" of applications where you don't want it to produce any real output (akin to a Unix command that you redirect output to /dev/null).

• Make "fake" input that looks like a file, but the file doesn't exist (if you are happy with constant-colored pixels).

The filename allows a REST-ful syntax, where you can append little doodads that specify things like resolution (of the non-existent file), etc. For example,

    foo.null?RES=640x480&CHANNELS=3

would specify a null file with resolution 640x480 and 3 channels. Token/value pairs accepted are

    RES=1024x1024        set resolution (3D example: 256x256x100)
    CHANNELS=4           set number of channels
    TILES=64x64          makes it look like a tiled image with tile size
    TYPE=uint8           set the pixel data type
    PIXEL=r,g,b,...      set pixel values (comma separates channel values)
    TEX=1                make it look like a full MIP-mapped texture
    attrib=value         anything else will set metadata!

Along the way, I also made some tweaks to ImageInput::create() to correctly disassemble the RESTful filename to figure out the extension (and thus which II plugin to use, without having to try them all).