Graphic quality of stamped sprites is very low

[thisandagain]

From internal playtest, 3/3/17.

Expected Behavior

The stamped sprites shouldn't be appreciably lower-res than the original sprite.

Actual Behavior

The graphic quality of the stamped sprites is really low.

Operating System and Browser

e.g. Mac, Chrome

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Moved from scratchfoundation/scratch-gui#204
/cc @jwzimmer

[thisandagain]

Related to #77