Fixing few GIMP scripts

Running a couple of GIMP scripts gave errors.

See

• https://www.gimp.org/docs/script-fu-update.html

The issues described in that link seem to be already fixed: there are other issues not stricly related to the change of the language, but likely to changes in GIMP (2.10.8).

E.g., having LAYER-MODE-NORMAL instead of NORMAL or BLEND-LINEAR instead of just LINEAR it's a good thing, but it breaks code using the “unprefixed” symbols.

Note: those scripts are installed, in my current (as of August, 2019) Ubuntu 19.04 distro, in /usr/share/gimp/2.0/scripts/ and GIMP's version is 2.10.8 as already written.

planet-render1-2.scm

• NORMAL becomes LAYER-MODE-NORMAL-LEGACY.

The use of the -LEGACY version seems logical. I haven't tried otherwise.

xtns-demartin-liquid-water.scm

• NORMAL becomes LAYER-MODE-NORMAL-LEGACY
• displace-type for plug-in-displace set to 1 (WRAP): 0 isn't in the range of accepted values.

xtns-lankinen-fiery-steel.scm

• NORMAL, OVERLAY and ADDITION become LAYER-MODE-*-LEGACY
• CUSTOM for gimp-blend becomes BLEND-CUSTOM
• LINEAR for gimp-blend becomes GRADIENT-LINEAR
• LINEAR for plug-in-bump-map locally defined as 0

xtns-demartin-shiny-painting.scm

• NORMAL as LAYER-MODE-*-LEGACY
• LINEAR for plug-in-bump-map locally defined as 0

xtns-gutteridge-dracula.scm

• NORMAL as LAYER-MODE-NORMAL-LEGACY (using define)

xtns-mosteller-multi-effect-text.scm

• plug-in-scatter-hsv is now plug-in-hsv-noise