tile-handler-empty: make it harder to dirty the empty common buffer

When the empty->tile was unreffed because its buffer was,
a tile of an other buffer might become the only 'owner' of
'common_buffer' which was possibly dirtied after unlocking
this tile.

By making every tile_handler_empty->tile sharing its
data with a never ureffed tile it will be harder to legally
dirty the 'common_buffer'
do not store default color in property-key metadata
operations: annotate plasma's properties with property keys

Use an ad-hoc made up "role" to specify the output size, this
probably has to change.
operations: annotate plasma's properties with property keys

Still using "role", but also some keys that feel much more permanent
to specify properties that are coordinates and distances in buffer
pixels.
operations: better keys for plasma
Port tile-glass to GEGL
paramspecs: use -G_MAXDOBULE - G_MAXDOUBLE as default double range
emboss: improve blurbs' compact property declarations
fractal-explorer: use gegl-op.h
checkerboard: use color-primary and color-secondary as property-key names

This replaces foreground-color and background-color; it is better if the
declaration of the property keys is focused on how things are from a GEGL
perspective; rather than using concepts belonging in some uses of GEGL.
grid: use gegl-op.h
vignette: add property meta-data
pixelize: add property meta-data
noise-reduction: use gegl-op.h
threshold: compact property meta-data
layer: compact property metadata
noise-rgb: compact property meta-data
plasma: compact property meta data
color: compact property meta data
panorama-projection: add output-extent annotations