Make it possible to use and create a custom BlurController

[behenate]

Why

Hi!
In expo-blur we would like to create our own version of PreDrawBlurController unfortunately a lot of stuff has been made package private. Making it impossible to do without creating a fork of the library.

How

I've split the PR into three commits:

• Allow setting up with a custom blur controller - Adds one more constructor to the BlurView, which accepts the desired blur controller. I also changed the other constructors to use it, and made the blurController protected so that views, which extend the BlurView can access it.

• Expose methods necessary for custom BlurControllers - The idea was to make it possible to copy contents of PreDrawBlurController into another package and still have it working. This mostly requires changes in the SizeScaler class where a lot of the methods were package private, and making the setContext method of the RenderEffectBlur public.

• Make it possible to extend PreDrawBlurController - I'm not sure if you will be ok with these changes, we can manage without them, but it would be quite a bit easier for us if we could extend PreDrawBlurController. Makes the class non-final and changes the access of some variables and methods to protected so inheriting classes have access to them.

The first two commits would be necessary for us to do what we want, the third one is nice to have.
Can you let me know if you would be ok with those changes?

Test Plan

Tested in the included test app and our react-native library.

[Dimezis]

Thanks for the PR and thoughtful description.
Out of curiosity, what's your use case for extending/changing the blur controller?

[behenate]

During screen transitions made by react-native-screens the blur view stops working. I'm suspecting that it is an issue with hardware acceleration during the transition, since AFAIK it's not possible to capture contents of hardware-accelerated layers. Our use case would be to detect the transition and pause blur updates to until they are finished. It's not optimal but should fix some of the most jarring issues.

We have to check it inside of the updateBlur method of the controller. Using setAutoBlurUpdate from an independent listener is not fast enough and we still do one invalid draw in that case.

[Dimezis]

Normally the Views would fall back to software rendering when needed, even if they have a hardware layer set.
You can test this by setting a hardware layer to any View in the sample app and see it still getting blurred.

Overall I'm fine with merging this PR, but maybe I could help investigate the root cause of your issue instead.
If you have an example project I could debug, I could take a look

[behenate]

@Dimezis Solving the root cause would be great, I'll check it, maybe my theory was wrong 🤔