Sticker Thumbnail Preview Transparency Issue

*This issue was originally created by [**@viasux**](https://github.com/viasux) at .*

### Background information

- **Dendrite version or git SHA**: dendrite 0.3.11-1(https://aur.archlinux.org/packages/dendrite/)
- **Monolith or Polylith?**: Monolith
- **SQLite3 or Postgres?**: Postgres
- **Running in Docker?**: No
- **`go version`**: go 2:1.16.2-1

### Description

Stickers sometimes have a non-transparent background (using https://github.com/maunium/stickerpicker)
I asked the stickerpicker dev about it and they told me it was most likely dendrite.

### Steps to reproduce

- Create a stickerpicker instance and implement in element
- Send stickers, most of them will have a black background, but some will have a transparent background

What is expected is that the background would be transparent.