null image input & output

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!

Author: lgritz

CMakeLists.txt

@@ -237,7 +237,7 @@ oiio_add_tests (
                 dither dup-channels
                 dpx ico iff
                 jpeg-corrupt-exif
-                png
+                null png
                 psd psd-colormodes
                 rla sgi
                 rational

src/libOpenImageIO/imageioplugin.cpp

@@ -243,6 +243,7 @@ catalog_plugin (const std::string &format_name,
     PLUGENTRY (iff);
     PLUGENTRY (jpeg);
     PLUGENTRY (jpeg2000);
+    PLUGENTRY (null);
     PLUGENTRY (openexr);
     PLUGENTRY (png);
     PLUGENTRY (pnm);
@@ -311,6 +312,7 @@ catalog_builtin_plugins ()
 #ifdef USE_OPENJPEG
     DECLAREPLUG (jpeg2000);
 #endif
+    DECLAREPLUG (null);
     DECLAREPLUG (openexr);
     DECLAREPLUG (png);
     DECLAREPLUG (pnm);

src/null.imageio/CMakeLists.txt

@@ -0,0 +1 @@
+add_oiio_plugin (nullimageio.cpp)