Benchmark fails occasionally due to imageDecodeTime being negative

[pastelmind]

Benchmark for ag-psd occasionally fails with the following error: imageDecodeTime must be a nonnegative number

This is probably happening because of the rather fragile (and broken) way we currently use to measure the image decode time, i.e. by subtracting the parse time (measured separately) from the total render time:

psd/packages/benchmark/src/bench.ts

Lines 101 to 102 in 2bda2cc

	imageDecodeTime:
	parseAndImageRenderEnd - parseAndImageRenderBegin - parseTime,

The benchmark app is long overdue a rewrite anyway; I'm thinking of writing it in React.