GR: Fix zscale support in surface plots (#5063)

Previously, `zscale` was being set to a Bool value (the result of the
set membership check). This caused a crash when querying the
`gr_z_log_scales` tuple on the next line, because that array is indexed
by symbols like :log10.

Given that the lines above assign the actual symbols to `xscale` and
`yscale`, it seems likely that `zscale` should behave the same.

Fixes: #5062

Author: JakubVanek

src/backends/gr.jl

@@ -1370,7 +1370,7 @@ gr_set_window(sp, vp) =
             if (yscale = sp[:yaxis][:scale]) ∈ _logScales
                 scaleop |= gr_y_log_scales[yscale]
             end
-            if needs_3d && (zscale = sp[:zaxis][:scale] ∈ _logScales)
+            if needs_3d && ((zscale = sp[:zaxis][:scale]) ∈ _logScales)
                 scaleop |= gr_z_log_scales[zscale]
             end
             sp[:xaxis][:flip] && (scaleop |= GR.OPTION_FLIP_X)