Performance. Unnecessary boxing in enum comparison.

[JimBobSquarePants]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of ImageSharp
• I have verified if the problem exist in both DEBUG and RELEASE mode
• I have searched open and closed issues to ensure it has not already been reported

Description

There's a few places in the code base (most notably png and gif formats) where I've incorrectly used enum.Equals(object). We should remove the boxing allocation by using enum == enum instead.

• ImageSharp version:
• Other ImageSharp packages and versions: 1.0.0-Beta6