Support "proper" address translation

[mattgodbolt]

jsbeeb/video.js

Lines 245 to 264 in a2cf276

	this.readVideoMem = function () {
	if (this.addr & 0x2000) {
	// Mode 7 chunky addressing mode if MA13 set.
	// Address offset by scanline is ignored.
	// On model B only, there's a quirk for reading 0x3c00.
	// See: http://www.retrosoftware.co.uk/forum/viewtopic.php?f=73&t=1011
	var memAddr = this.addr & 0x3ff;
	if ((this.addr & 0x800) || this.isMaster) {
	memAddr |= 0x7c00;
	} else {
	memAddr |= 0x3c00;
	}
	return this.cpu.videoRead(memAddr);
	} else {
	var addr = (this.scanlineCounter & 0x07) | (this.addr << 3);
	// Perform screen address wrap around if MA12 set
	if (this.addr & 0x1000) addr += this.screenAdd;
	return this.cpu.videoRead(addr & 0x7fff);
	}
	};

c.f. http://beebwiki.mdfs.net/Address_translation