Set in & outSourceMap directly, not via merge

Previously, input sourcemaps would gradually accumulate more and more `sources` / `sourceContents` entries as files passed through the stream, since each one was being merged into `options`. When the sourcemaps got merged, the sources/sourceContents would have the same length as the original maps, but the contents would be wrong for all but the first streamed file.

This sets the values directly on `options`, which avoids this accumulation. An alternate approach would be re-creating the options for each stream, but this seemed more efficient.