[p5.strands] Add texCoord to finalColor block

[nbogie]

Increasing access

Newcomers to shaders would be able to make simple unlit material shaders sooner, with only finalColor (and, say, noise() on texCoord).
In contrast, currently, I think they need to understand how to use shared variables to propagate the texCoord from pixelInputs, and need to know that it even exists there.

Most appropriate sub-area of p5.js?

• Accessibility
• Color
• Core/Environment/Rendering
• Data
• DOM
• Events
• Image
• IO
• Math
• Typography
• Utilities
• WebGL
• Build process
• Unit testing
• Internationalization
• Friendly errors
• Other (specify if possible)

Feature enhancement details

Instead of this for a material shader:

before

let coord = sharedVec2()
pixelInputs.begin();    
coord = pixelInputs.texCoord;
pixelInputs.end();

finalColor.begin();
let coord = finalColor.texCoord;
finalColor.set(noise(coord.x, coord.y));
finalColor.end();

...we could instead write this:

after

finalColor.begin();
let coord = finalColor.texCoord;
finalColor.set(noise(coord.x, coord.y));
finalColor.end();

This assumes there is no situation where it texCoord for finalColor would be meaningless or ambiguous - I don't know.

Conversation in discord:

https://discord.com/channels/836700474425475088/1384610702467272785/1485021371607154749

[YuktiNandwana]

Hi @nbogie and @davepagurek! I've been exploring the new p5.strands architecture for my GSoC proposal and I'd love to take this on. Making shaders more accessible for beginners aligns perfectly with what I want to contribute to p5.js.

I'm currently looking into the WebGL module to see where the default properties for the finalColor block are defined. Could I be assigned to this issue?