Krakout: Reset the title-screen credits marquee to the start of its message

C3C1: Reset the title-screen credits marquee to the start of its message

Resets the credits scroller so the message restarts from its first character, with the load column blanked. It then runs on into the first scroll step.
The marquee is one character row tall at screen row 19, over columns 8–23. New glyphs are dropped into the rightmost cell (the load column) and scroll leftward into view. That cell is black ink on black paper, so a freshly fed glyph stays hidden until it scrolls into the next cell.
credits_scroll_init	C3C1	LD HL,ATTR_BASE+$277	Blank the load column (rightmost marquee cell, row 19 column 23): black ink on black paper hides the glyph fed there until it scrolls left.
	C3C4	LD (HL),$00
	C3C6	LD HL,credits	Point the read pointer at the first character of the credits message.
	C3C9	LD (credits_read_ptr),HL
	C3CC	XOR A	Reset the pixel-phase counter to 0.
	C3CD	LD (credits_scroll_phase),A	Reset the pixel-phase counter to 0.

Routines