[BREAKING] More general instancing

[mvaligursky]

Fixes #4878

Marked as BREAKING, as in case the user modified transformVS chunk, this needs to be updated to the new format. But no public API has been changed otherwise, and the default instancing works exactly as before.

https://x.com/ValigurskyM/status/1819367152614260934

This PR adds a user exposed and more generic version of hardware instancing. Details:

• an instancing part of the transformVS chunk has been moved to transformInstancingVS chunk. This implements the existing instancing, where 4 x Vec4 are passed in by the vertex buffer, representing a world matrix.
• this chunk is available as an include to ShaderMaterial, allowing instancing support by it
• this chunk can be overwritten allowing custom attributes and implementation - user can also pass in any attributes like non related to transform, say per instance color or texture offset or anything else.

Example of custom instancing for StandardMaterial:

const material = new pc.StandardMaterial();

// set up additional attributes needed for instancing
material.setAttribute('aInstPosition', pc.SEMANTIC_ATTR12);
material.setAttribute('aInstScale', pc.SEMANTIC_ATTR13);

 material.chunks.transformInstancingVS = `
    // instancing attributes
    attribute vec3 aInstPosition;
    attribute float aInstScale;

    // function to generate world matrix
    mat4 getModelMatrix() {
        return mat4(
            vec4(aInstScale, 0.0, 0.0, 0.0),
            vec4(0.0, aInstScale, 0.0, 0.0),
            vec4(0.0, 0.0, aInstScale, 0.0),
            vec4(aInstPosition, 1.0)
        );
    }
`;

Example of custom instancing for ShaderMaterial. Shader variations for instancing are created automatically if instancing is set up on the mesh instance using this material.

const material = new ShaderMaterial({
    uniqueName: 'GoochShader',
    vertexCode: /* glsl */ `
    
    #include "transformCore"
    
    #if INSTANCING
        // add instancing attributes we need for our case - here we have position and scale
        attribute vec3 aInstPosition;
        attribute float aInstScale;
        // instancing needs to provide a model matrix, the rest is handled by the engine when using transformCore
        mat4 getModelMatrix() {
            return mat4(
                vec4(aInstScale, 0.0, 0.0, 0.0),
                vec4(0.0, aInstScale, 0.0, 0.0),
                vec4(0.0, 0.0, aInstScale, 0.0),
                vec4(aInstPosition, 1.0)
            );
        }
    #endif
        
    // the rest is the same as without instancing
    void main() {
        mat4 modelMatrix = getModelMatrix();
        vec3 localPos = getLocalPosition(vertex_position.xyz);
        vec4 worldPos = modelMatrix * vec4(localPos, 1.0);
        gl_Position = matrix_viewProjection * worldPos;
    },
    fragmentCode: // whatever,
    attributes: {
        vertex_position: SEMANTIC_POSITION,
        vertex_normal: SEMANTIC_NORMAL,
        aUv0: SEMANTIC_TEXCOORD0,

        // instancing attributes
        aInstPosition: SEMANTIC_ATTR12,
        aInstScale: SEMANTIC_ATTR13
    }
});

New examples

ShaderMaterial using instancing:

Custom StandardMaterial instancing: