corelib: PUTPIXEL can draw color 0 again

lib/corelib.s

@@ -754,6 +754,9 @@ PUTPIXEL_4BPP:
 	STORE.B FB_WA	; set as write and read addresses
 	STORE.B FB_RA
 
+	LOAD PUTPIXEL_COLOR
+	CBRANCH.Z PUTPX_CLR	; color 0 is special case
+
 	; create pixel data from color value in
 	; leftmost pixel data bits (31-28)
 	LOADC 0
@@ -775,12 +778,29 @@ PUTPIXEL_4BPP:
 	OR			; OR in new pixel bits
 	STORE.B FB_IO		; write new pixel data word to vmem
 
+PUTPX_XT:
 	LOAD PUTPIXEL_BPSAV
 	STOREREG BP
 
 	FPADJ PUTPIXEL_FS
 	RET
 
+PUTPX_CLR:
+	LOADCP $F0000000	; mask for leftmost pixel
+	STORE.B FB_SHIFTER	; shift accordingly
+	LOAD PUTPIXEL_X
+	STORE.B FB_SHIFTCOUNT
+
+	LOAD.B FB_SHIFTER	; get shifted value
+	NOT			; invert for real mask
+	LOAD.B FB_IO		; get background pixels
+	AND			; clear pixel with mask
+	STORE.B FB_IO		; no need to OR in new pixel, just store to vmem
+
+	BRANCH PUTPX_XT
+
+
+
 ; draw a line between two points
 ; parameters: x0, y0, x1, y1, color
 	.EQU DL_X0 0