Implement pixel-perfect/subpixel rendereing toggle for FlxCamera

This change adds `pixelPerfect` to `FlxCamera`, allowing you to set pixel-perfect rendering per-camera, and changes instances of `pixelPerfectRender` to use a `pixelPerfect`-aware check, while preserving the existing behavior.

Notes:
I had spoken to Gama11 about this in IRC, and was recommended an approach similar to antialiasing (`antialiasing || camera.antialiasing`).

If I had done `Camera.pixelPerfect && pixelPerfectRender`, it would exclude the case where you had set `Camera.pixelPerfect = false`, but wanted to set `pixelPerfectRender = true` on your object; this allows `pixelPerfectRender` to take precedence regardless of what value `Camera.pixelPerfect` has been assigned.