Fix POINT coordinate mapping, remove orphaned palette array, expand tests

Apply coordinate mapping (VIEW/WINDOW) to POINT(x,y) function so it
returns correct pixel values when WINDOW is active. Remove unused
palette[] array from graphics.c (Sixel encoder uses palette_map[]
directly). Expand view_window.bas test to cover WINDOW SCREEN mode,
VIEW+WINDOW combination, and PMAP inverse mapping. Fix CI test count
in docs.

tests/expected/view_window.expected

@@ -4,4 +4,13 @@ PMAP(100,0)= 639
 PMAP(100,1)= 0
 PMAP(0,2)= 0
 PMAP(199,3)= 0
+SCR PMAP(0,1)= 0
+SCR PMAP(100,1)= 199
+SCR PMAP(50,0)= 319.5
+SCR PMAP(0,3)= 0
+SCR PMAP(199,3)= 100
+VP PMAP(0,0)= 100
+VP PMAP(10,0)= 300
+VP PMAP(0,1)= 150
+VP PMAP(10,1)= 50
 VIEW/WINDOW/PALETTE OK