The plugin ignores any sourcemap chains from other plugins transformations

[rproserpio]

When collecting the processed css in the transform hook, rollup-plugin-postcss loses the possible sourcemap chain with the transformations of previous plugins.

E.g if the input .css is generated from a .svelte file, the .svelte to .css transformation is lost in the final sourcemap (see sveltejs/rollup-plugin-svelte#166 ).

A fix may be collapsing the previous sourcemaps with Rollup this.getCombinedSourcemap() as an input of the Postcss transformation.