* Added new gray data types adding those to the variants and updating all the code necessary for them
* Added basic SSE to the image compare method, (only for RGBA) must be enabled with the -DSSE_MATH flag this is not yet put into the build process in any location.
* Fixed the resulting image for some TIFF visual tests, most likely they were incorrect due to fixes in TIFF reader
* Added some MAPNIK_DECL where necessary to grid rendering.
* Added support for more data types in GDAL plugin with grayscale images.
* Added views for all the new gray data types
* Updated python bindings for new gray data types.
Ref #2681
