Trim inline sourcemaps emitted from other rollup plugin

I'm using svelte and when I enable `emitCss` option, rollup-plugin-postcss should receive css "files" with inline sourcemap.

`rollup.config.js`
``` javascript
...

plugins: [
svelte({
...
// Emit CSS as "files" for other plugins to process
emitCss: true
}),

postcss({
extract: true,
sourceMap: true
})

...
]

...
```

But the output has 2 sourcemaps, the inline sorcemap from the emitted svelte css files and from the postcss generated sourcemap. It would be nice to trim the emitted inline sourcemap and extract it to the specified `.map` file, reducing the file size significantly.