- Add missing doc/webp_figures.gnu fronm upstream git repo

- Update to gnuplot 6.0.0
* Japanese documentation updated
(missing file title-ja.tex, build dependencies for mingw)
* Support building against Qt6
(./configure falls back to Qt5 if the Qt6 libraries are not found)
* New configure option --enable-stable-sort
1) Use mergesort rather than qsort if the system provides it.
2) If neither mergesort nor glibc stable qsort is available,
modify internal gnuplot structures to contain a secondary sort key
* Render color gradient in colorbox as a pixel array (cairo, qt terminals)
* "reset session" restores initial state of all linetype properties
* postscript terminal now handles pixmaps (Bug #2644)
* New plot style "with contourfill" handles 2D and 3D contour plots with
discrete z-ranges indicated by solid fill colors.
* FIX do not clobber a datablock if its name is encountered unexpectedly
* FIX cairo terminals: ignore zero-size polygons
* FIX svg terminal now correctly tracks pattern-fill color
* FIX integer overflow/wraparound when polygon is clipped to plot border
* FIX do not mangle variable point properties in polar mode Bug #2650
* partial support for replotting and mousing multiplots (finally!)
* fixes build error for postscript terminal when libgd and cairo are not present
* strip trailing CR from function blocks read from a dos file Bug #2653
* use 64-bit integers for iteration
* "unset pointintervalbox" disables the box rather than setting it to default size
* keyentry '...' can be used to place a secondary title in the key
* fix use of stacked histograms with variable number of data columns
- Replotting and mousing multiplots
o now working much more comprehensively