Epson resolution

Epson GT-1500 supports generic 'resolution' option, which only supports one value 600, and specific settings for x and y axes, which support a number of different settings. Current code results is painfully slow scan, as it always uses 600 dpi. This fix speeds up scans at usual 150 dpi by factor of 4 and also allows to access (in principle) hardware-driven resolutions from 150 up to 2400 dpi.