fix: disable alpha channel in canvas's framebuffer
fix: disable alpha channel in canvas's framebuffer

The alpha channel of the canvas's framebuffer allows blending with other elements in the browser,
but this functionality is not needed by Neuroglancer, and was leading to inconsistent results on
different browsers.

janelia-flyem/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,google/neuroglancer,google/neuroglancer,seung-lab/neuroglancer