GLFunctorDrawable: skip alpha filtering

When drawing a GLFunctorDrawable into an AlphaFilterCanvas, there is no
gpu device, so topLayerBackendRenderTarget (and therefore GetFboDetails)
fails. Applying the alpha would require an extra layer and a performance
cost, so draw directly to the underlying gpu canvas.

This effectively matches the VkFunctorDrawable fix in
I27040347dc25c799b4e75f50526f426e9e33b663.

Bug: 208629203
Test: Iab6f59411310fe4f5ea4ccc3bf386dcc6f2007b1
Change-Id: I9b8f0a357b9349fbb568b30dd18b0c150065f13c