buffer: iterator, add linear buffer special case
buffer: change default tile size to 128x256

For a three layer large GIMP test document, the debug benchmark of
revalidating the projection goes from 1.65 seconds to 1.14 seconds after the
change.
python: remove bogus test with hard-coded gegl config values
tests: Fix shadow in gegl logo composition.
buffer: Clarify the docs for gegl_buffer_linear_open
tests: Fix gegl logo reference composition.
buffer: new default tile size 512x64
buffer: fix linear buffer fast path
buffer: make linear open buffer test pass
buffer: make direct access in GeglBufferIterator more picky
2x2-downscale: unroll component loop for 1-4 components
buffer: disable linear fast path in gegl-buffer-iterator

See comment left with short-circuit of condition for reason.
operation_get|list_key(s): move to be included from gegl.h
docs: add gir parsing capabilities do js doc viewer
buffer: remove unneeded copy in nearest sampler
buffer: short-circuit nearest sampler with a 1x1 gegl_buffer_get

The GeglSampler base-class is micro optimized for affine transformations at
the moment, for now, this short-circuit of the nearest sampler implementation
avoids a lot of redundant elbow-room caching.
buffer: make nearest sampler fast path catch more cases

The logic to perform conversions if the buffers format is not same as desired
are already in place.
perf: update performance testing framework
perf: set nearest sampler correctly in nearest rotate
2x2-downscale: fix regression with integer overflowing sums