fix: allow density values above 2400 dpi

[stefanprobst]

currently, density is capped at 2400dpi. when e.g. converting svg images with a small viewBox or small width/height values to png images, this can be not enough to produce crisp results.

according to this comment librsvg supports values up to 100000, which is what this PR allows.

my main motivation here is actually not even image quality but file size, i'm seeing 100243 bytes with the density cap, vs 22262 bytes without.

(not sure though if this has any consequences i'm not aware of, or what the original reason for the 2400dpi cap was)

[coveralls]

Coverage remained the same at 100.0% when pulling b794669 on stefanprobst:fix/density-dpi into d406cb6 on lovell:master.

[lovell]

Thank you very much!