Update rust-layers to fix empty BufferRequest failures

rust-layers will now avoid sending empty buffer requests which can
cause failures when Servo tries to create empty platform surfaces.

Fixes #4060.
auto merge of #4074 : mrobinson/servo/empty-tiles, r=jdm

rust-layers will now avoid sending empty buffer requests which can
cause failures when Servo tries to create empty platform surfaces.

Fixes #4060.