Ensure that effects are scaled for pixmaps that are scaled by a fractional amount

This fixes two bugs in `Generator.prototype.getPixmapParams`:

1. The expressions that were used to define `paddedOutputHeight` and `paddedOutputHeight` mixed up width and height; and
2. The test for scaling uniformity, which determines whether or not effects are scaled, occurred after a ceiling computation, which in some cases caused effects scaled by a non-integral factor to be considered non-uniform. 

Addresses adobe-photoshop/generator-assets#200. I've also added a new regression test to verify this in  adobe-photoshop/generator-assets-automation@89241e359cdda7217f736d660f6f82f48d258ed6. 

CC @joelrbrandt