Implement pixel-perfect/subpixel rendereing toggle for FlxCamera

[schonstal]

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.

[gamedevsam]

Glorious!!!

[gamedevsam]

This should just return Bool, not Null<Bool>.

[schonstal]

I made it nullable so isPixelPerfect could see if a value had been set.

[gamedevsam]

It's a little odd, but makes sense. I have no idea why you'd want some sprites to be pixel perfect and others to not be pixel perfect, but it doesn't hurt to have that capability...

[Gama11]

That can make sense on Flash. pixelPerfect forces them to be draw()n, which is rather expensive, so you might only want the most important stuff (player char etc) to use subpixel-rendering.

[Gama11]

Nice! I like the Null<Bool> approach, it's in a way more intuitive than the antialiasing one... I think I'm going to change that to use the same.

I thought about implementing this the other way around - have the camera's pixelPerfect be a Null<Bool> and null by default, and if you set it, it overrides the setting of all sprites on it. But I think this approach makes more sense.

I don't like that you chose a different name for camera.pixelPerfect (as opposed to pixelPerfectRender).

[Gama11]

This doesn't really work in its current state... It would require changing isSimpleRender() to isSimpleRender(Camera), since pixelPerfectRender affects the results of isSimpleRenderBlit():

public function isSimpleRenderBlit():Bool
{
    return ((angle == 0) || (bakedRotationAngle > 0)) && (scale.x == 1) && (scale.y == 1) && (blend == null) && pixelPerfectRender;
}

[schonstal]

isSimpleRenderBlit is used in stamp, which I don't think should care about which camera the sprite is on?

Null evaluates to false so this might make stamp behave strangely.

Maybe it should be changed to:

public function isSimpleRenderBlit():Bool
{
    return ((angle == 0) || (bakedRotationAngle > 0)) && (scale.x == 1) && (scale.y == 1) && (blend == null) && (pixelPerfectRender == null || pixelPerfectRender);
}