How software rendering works in this embedder

All memory that is allocated for graphics surfaces on Fuchsia must come from sysmem.

Normally, software rendering in the Flutter embedder uses a surface that Flutter allocates itself using Skia. We don't want this default behavior because we need our surface to come from sysmem. As a result we provide our own callback for allocating software surfaces to the embedder (see FuchsiaAcquireSoftwareSurface).

SoftwareSurface is where we interact with sysmem to get the buffer for software rendering. We then register this buffer with Flatland, getting a token that we can use to turn this buffer into a Flatland image.

The lifecycle currently goes:

• At app startup, Flutter acquires a software surface from us.
• Flutter then repeatedly asks us to present that software surface. Reusing the same software surface is fine from frame to frame as long as the window has not been resized.

Future work

• How does window resizing work?
• When does Flutter acquire a new surface for software rendering?