Any way to clear the draw buffer with NVG?

I’m working on drawing a wasm gauge on top of a javascript gauge to provide
additional symbology to Bing maps. I haven’t discovered a way to clear the
draw buffer in webassembly with the gauge api when using nanovg. I’ve tried
using the low level api function FSRenderFlush with the context provided by
the gauge_callback before calling nvgCreateInternal, but that always ends up
crashing the game. When just displaying a wasm gauge, the solution would be to
call nvgFill with a black nvgFillColor on every PRE_DRAW, but that just
completely hides the javascript gauge behind it. I’ve gone as far as creating
a custom WasmInstrument class so I could set the opacity of the wasm gauge to
0.5, which partly works, but when the map becomes cluttered, or when
displaying something white, it overpowers the wasm gauge. I understand my use-
case is kind of one-off, but I think having access to clear the draw buffer
could have many benefits for people using the gauge api.