Add an option to use linearToOutputTexel.

[dli7319]

When a render target is set, threejs automatically change their shaders to output in Linear color space using linearToOutputTexel: https://github.com/mrdoob/three.js/blob/e04b9f7bd7f5b17103339d343168bfab2d6e0ace/src/renderers/WebGLRenderer.js#L2198

However, with Sparkjs, we need to manually set encodeLinear or else we get a washed out image like this:

With this option, we now have the option of setting sparkRenderer.defaultView.useLinearToOutputTexel = true; and the Spark fragment shader will automatically output in the correct color space the same way that three.js's built-in shaders do.

[dli7319]

See https://github.com/dli7319/spark/blob/linearToOutputTexelTesting/examples/hello-world/index.html for testing

[mrxz]

Changes seem correct, but I'm not too fond of introducing a new property. And there's something wasteful about converting the colour from sRGB to linear just to convert it back.

How about automatically setting the encodeLinear uniform in SparkRenderer based on the currentRenderTarget? That's what I did in #196, see Splat.ts#L311-L319.

[dli7319]

I see your PR has it covered. Thanks.