Very small Canvas sizes may produce erroneous pixels

If the number of pixels is very small, some visual bugs may appear. Try, e.g.,
`C = canvas(Float64, m, n); windowscale!(C,100)`
for m=n=5 to potentially reproduce the error.
For m=n=4, there seems to be no issue. The root cause seems to be that there is a minimum (or some kind of) size requirement for textures on the GPU. A potential solution would be to allocate at least say 32x32 textures and just show only the required pixels.