image create crashes browser if parameter is not integer

[jwunderl]

Describe the bug

image create crashes browser if parameter is a double

To Reproduce Steps to reproduce the behavior:

1. Go to https://makecode.com/_XHAFjcRy1Mgf
2. See error

Expected behavior

should not crash browser, probably just needs a pair of | 0's on the params in https://github.com/microsoft/pxt-common-packages/blob/master/libs/screen/sim/image.ts#L14 / maybe hw side as well

worth noting that this looks like it's just when wh does not make an integer (e.g. here it's 1.5 * 1.5) -- here with 1.52 works without crashing https://makecode.com/_iVu317FgYh8w, so maybe core issue is something like the buffer allocation logic having an equals check instead of a comparison or something?

maybe also worth just returning undefined instead of crashing when w / h is negative, but I would expect something to die in that case eventually anyways