The plugin ignores any sourcemap chains from other plugins transformations

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 https://github.com/sveltejs/rollup-plugin-svelte/issues/166 ).

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