gnuplot: updated to 5.4.2

Changes in 5.4.2

NEW in "using" context, $# means "number of columns in current data line"
NEW define on program entry: I = sqrt(-1) = {0.,1.}
NEW support input data that uses epidemiological date-week formats
NEW optional fillcolor for "set key opaque"
NEW provide and document use of a plugin to provide uigamma function
CHANGE much improved domain and precision for igamma function
CHANGE generate only one key entry per multi-dataset input file
CHANGE (Windows): attempt to convert unicode on command line to UTF-8
CHANGE clip `with image` to plot boundary in 2D plots
CHANGE In gprintf format "%c", add no space for a null prefix
CHANGE Make auto-placement of left border less sensitive to plot size
FIX time formats %U %W
FIX left-justification of titles in splot with user-specified key position
FIX clipping of filled polygons that have only 1 or 2 vertices
FIX "set tics front" unexpectedly made all grid lines invisible
FIX more terminals support requested linewidth for textbox borders
FIX indexing of multiple matrix data sets in a single input file
FIX do not inject extra '\n' into input stream after 'set term'
FIX cairo terminals: Do not lose font info during multi-line enhanced text
FIX cairo terminals (Windows): ignore "text scaling" from user screen settings
FIX pm: better mousing and font handling
FIX win: instabilities associated with changing window size
FIX wxt: use of multiple plot windows from single-threaded wxt terminal
FIX qt: prevent creation of zombie gnuplot_qt process

(adam)