widgets: Fix label background on Curves tool

Resolves #11702
In 2.10, the x/y label background on the Curves tool used
GtkStateType enums to get the widget's background color.
These enums were depreciated in GTK3, and during the
port the gdk_cairo_set_source_rgba() command was left
out. This resulted in the current line color being used instead,
making it hard to read the label in some instances.

This patch restores the Cairo color update command, using
the gridline color as an equivalent to the background color
from 2.10.

app/widgets/gimpcurveview.c

@@ -782,6 +782,7 @@ gimp_curve_view_draw (GtkWidget *widget,
 
       cairo_push_group (cr);
 
+      gdk_cairo_set_source_rgba (cr, &grid_color);
       cairo_rectangle (cr, x + 0.5, y + 0.5, w, h);
       cairo_fill_preserve (cr);