Fix grdview Qt backend behavior with -Qt in 3-D views

[Esteban82]

Description

When running the script below, the following error is triggered:

grdview [ERROR]: Categorical data (as implied by CPT) cannot be interpolated and require -T or just -Qm.
This PR adds an exception that allows using -Qt with categorical CPTs when grdview is run with a 3-D projection (-JZ).

Full script:

gmt begin ex05_3d png
 	gmt makecpt -Ccategorical -T0/4/1 -F+cOceanic,Land,Lakes,Islands,Ponds
	gmt grdview @earth_relief -G@earth_mask -JM15c -JZ5c -B -Bza -BSEwnZ -N-500+gwhite -R-58/-54/-36/-32/-500/500 -p120/30 -C -Qt100i
gmt end

See details and the expected output figure in #8855 (comment)

[joa-quim]

Our CI tests are in a miserable state. Each day they fail for a different reason.

[Esteban82]

Our CI tests are in a miserable state. Each day they fail for a different reason.

Ok, I will merge it.