Handle rotation and mirroring in ElementImage
Closes #840 (closed)
This affects the three modals for creating elements, editing elements, and editing transcriptions.
When using both rotation and mirroring at once and drawing elements that are very thin like text lines, the display gets trimmed, seemingly because the viewBox is not calculated properly. I spent a while trying to debug this unsuccessfully and mirroring is not commonly used so this can be ignored for now.
Edited by Erwan Rouchet