refactor: use Params struct for imageprocessor constructor

Rename NewImageProcessor(maxInputBytes) to New(Params{}) with a Params
struct containing MaxInputBytes. Zero-value Params{} uses sensible
defaults (DefaultMaxInputBytes). All callers updated.

Addresses review feedback on PR #37.

internal/imgcache/service.go

@@ -82,7 +82,7 @@ func NewService(cfg *ServiceConfig) (*Service, error) {
 	return &Service{
 		cache:           cfg.Cache,
 		fetcher:         fetcher,
-		processor:       NewImageProcessor(maxResponseSize),
+		processor:       New(Params{MaxInputBytes: maxResponseSize}),
 		signer:          signer,
 		whitelist:       NewHostWhitelist(cfg.Whitelist),
 		log:             log,