a68b138 made draw_tab() derive all four color roles from tab_colors(),
so it stopped reading the per-tab colors that `kitten @ set-tab-color`
sets. The ssh wrapper (__kitty_tab_set) still set them, but the renderer
discarded them and every remote tab drew in the #2e84e6 accent.
Resolve the drawn colors through DrawData.tab_fg/tab_bg on the already
role-substituted title_draw_data: those return the per-tab override when
one is set and the accent roles otherwise. Theme-provided tab colors
still cannot leak in, so the theme-independence fix is preserved.
Extend tests/test_tab_bar_colors.py with the override case; it fails
against the old renderer (got the accent, wanted the host color).
