fix(webgl/dynamic_shader): fix incorrect use of encodeContext in place of getContextKey
fix(webgl/dynamic_shader): fix incorrect use of encodeContext in place of getContextKey

This does not affect any existing uses, but will affect uses in subsequent commits.

google/neuroglancer,google/neuroglancer,google/neuroglancer,google/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer