Fix early buffer flush in image painter

[RblSb]

for (i in 0...300)
	g.drawImage(Assets.images.img, 0, i);
for (i in 0...1000)
	g.drawImage(Assets.images.img2, 500, i / 100);

Currently instead of drawing 2 drawcalls with 1800 + 6000 triangles, g2 draws 3 with 1800 + 5394 + 606, because first drawImage call moved bufferStart, and since bufferIndex is not relative to bufferStart, buffer end check is incorrect.

Also drawImageScale is unused, so i removed it.