Add NativeEncoding plugin

[alexchuber]

This PR introduces the NativeEncoding plugin, which provides native image encoding capabilities to JavaScript. Raw pixel data can be encoded into standard image formats (PNG, JPEG, WebP, etc.), serving Babylon.js's needs for texture export.
See the README for more info.

As this PR is focused on the plugin (and some smoke tests for it), the following will be delegated to future PRs:

• Adding example use cases to the Playground
• Adding example use cases to the visual test suite
• Using the plugin from TestUtils

[CedricGuillemet]

TestUtils plugin has a basic png writer (

BabylonNative/Plugins/TestUtils/Source/TestUtils.cpp

Line 23 in a018058

void TestUtils::WritePNG(const Napi::CallbackInfo& info)

) that should be replaced by this PR change. It's being used by https://github.com/BabylonJS/BabylonNative/blob/master/Apps/Playground/Scripts/validation_native.js

[alexchuber]

TestUtils plugin has a basic png writer that should be replaced by this PR change.

Totally. Will do this in a follow-up PR.