Fix the text transparency ramp during the rise phase (-Et+r) for labels

[Esteban82]

Done by Claude Sonnet 5

Description

The rise-phase transparency was incorrectly computed using PSEVENTS_DECAY instead of PSEVENTS_RISE. Since labels do not have a decay phase, this resulted in a division by zero and generated Inf/NaN transparency values. Consequently, pstext fell back to fully opaque text and issued a warning as described in #8328

Testing

This can be reproduced with:

echo 20 6.5 0 1 Red Label > red.txt
gmt events red.txt -T-0.6 -R-20/40/1/9 -JX20c/10c -Et+r1 -Mt100 -F+f18p+jBC -L -png test

Fixed:

With this fix, the generated frame shows a partially transparent label (~60% transparency).

Without fix:

Without this fix, the label is rendered fully opaque and GMT reports a warning about an invalid transparency value.

Fixes #8328

[joa-quim]

Seems pretty harmless if it woks.

[Esteban82]

It is also coherent with the funcion for symbols

	x = psevents_ramp (GMT, Ctrl, PSEVENTS_SYMBOL, PSEVENTS_RISE, t, t_now);	/* Ramp function */