Fix the sound timer, sprite wrapping under clip quirks, the quirked jump address, and Fx0A waiting for release

[HarianthK]

I ran Timendus' test suite through Chip8CPU with no window, using a small stand-in for pygame and a plain screen object, and found four things. Each is fixed here with a unit test.

Sound timer. decrement_timers had self.sound -= 1 if self.delay > 0 else 0, so the sound timer only counted down while the delay timer was running. A program that sets only the sound timer buzzed forever. One character.

Clipping. With clip_quirks on, a sprite whose start position lay past the edge was not drawn at all, because the position was only wrapped when clipping was off. The start always wraps around the screen; clipping only decides what happens to the part that hangs over. That is the ERR2 the quirks test shows on its clipping row. Fixed in both draw_normal and draw_extended.

Quirked jump. With jump_quirks on, Bxnn added only the low byte of the address to Vx, so jump0 0xE00 with vE at 0x9C landed at 0x09C instead of 0xE9C. The whole twelve bit address counts; its top nibble both names the register and is part of the address. The existing unit test encoded the low byte reading, so it changes with this, with the jump row of the quirks test as the reference.

Fx0A. Execution resumed when a key went down. The original hardware moves on when the key is released, and the keypad test's third check reports NOT RELEASED for this. The key is now remembered on press (decode_keypress_and_continue) and the register written on release (decode_keyrelease_and_continue, called from a new KEYUP branch in the emulator loop).

Results

Test	Before	After
Quirks as CHIP-8 (logic_quirks, clip_quirks)	clipping ERR2	every row but display wait, which is not implemented
Quirks as SUPER-CHIP (shift, index, jump, clip)	crashed on an unknown opcode at 0x09C	every row
Quirks as XO-CHIP	clipping ERR2	every row
Keypad, Fx0A	NOT RELEASED	ALL GOOD
Opcode and flags tests	pass	pass

python -m unittest passes, 140 tests including the five new ones. The harness that ran the suite is a throwaway and not included.