Remove all dialyzer opt-outs and fix the defects they masked

[hlindset]

Removed all @dialyzer attributes and the .dialyzer_ignore_warnings file. 27 were stale, 5 (+ the ignore file) masked defects.

• Image.drop_shadow/2 now validates before deriving :dy, and reports reason: :invalid_option. Only :opacity and :sigma are validated. :dx, :dy and unknown options are untouched here (out of scope), and will instead be fixed by a move to an Image.Options.DropShadow module in a follow-up
• Image.Math.cos/1 and sin/1: added {:error, Image.error()} return type
• Eight math operators gained Vimage.t() in its type (they all delegate to non-bang function that accept it)
• Added success path tests for the image + pixel shapes of all 11 bang operators

Breaking changes (ish)

• Image.drop_shadow/2 errors instead of raising on a non-numeric :sigma. ArithmeticError becomes {:error, %Image.Error{}}
• Image.drop_shadow/2's :opacity and :sigma errors carry reason: :invalid_option. value now set to {:opacity, value} and {:sigma, value}