Fix: Support WebGL2 MRTs

[Determinated738]

This PR fixes a bug where the OverrideDrawBufferEnum helper in src/native/webgl.cc lacked a default return case.

Previously, passing any color attachment index greater than 0 resulted in undefined behavior/garbage values being passed to the driver. This prevented Multiple Render Targets from functioning correctly in WebGL2 contexts.

Minimal code to recreate the bug:

import createGL from "gl";

const gl = createGL(2, 2, { createWebGL2Context: true });
gl.getExtension("EXT_color_buffer_float");

const tex0 = gl.createTexture();
gl.bindTexture(gl.TEXTURE_2D, tex0);
gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA32F, 2, 2, 0, gl.RGBA, gl.FLOAT, null);

const tex1 = gl.createTexture();
gl.bindTexture(gl.TEXTURE_2D, tex1);
gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA32F, 2, 2, 0, gl.RGBA, gl.FLOAT, null);

const fbo = gl.createFramebuffer();
gl.bindFramebuffer(gl.FRAMEBUFFER, fbo);

gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, tex0, 0);
gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT1, gl.TEXTURE_2D, tex1, 0);

gl.drawBuffers([
    gl.COLOR_ATTACHMENT0,
    gl.COLOR_ATTACHMENT1
]);

const vs = `#version 300 es
    in vec2 pos;
    void main() {
        gl_Position = vec4(pos, 0.0, 1.0);
    }`;

const fs = `#version 300 es
    precision highp float;

    layout(location=0) out vec4 out0;
    layout(location=1) out vec4 out1;

    void main() {
        out0 = vec4(1.0, 0.0, 0.0, 1.0);
        out1 = vec4(0.0, 1.0, 0.0, 1.0);
    }`;

function compile(type, src) {
    const s = gl.createShader(type);
    gl.shaderSource(s, src);
    gl.compileShader(s);
    return s;
}

const prog = gl.createProgram();
gl.attachShader(prog, compile(gl.VERTEX_SHADER, vs));
gl.attachShader(prog, compile(gl.FRAGMENT_SHADER, fs));
gl.linkProgram(prog);
gl.useProgram(prog);

const buf = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, buf);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
    -1, -1,
    1, -1,
    -1, 1,
    1, 1,
]), gl.STATIC_DRAW);

const loc = gl.getAttribLocation(prog, "pos");
gl.enableVertexAttribArray(loc);
gl.vertexAttribPointer(loc, 2, gl.FLOAT, false, 0, 0);

gl.viewport(0, 0, 2, 2);
gl.drawArrays(gl.TRIANGLE_STRIP, 0, 4);

function read(att) {
    gl.readBuffer(att);
    const out = new Float32Array(4);
    gl.readPixels(0, 0, 1, 1, gl.RGBA, gl.FLOAT, out);
    return Array.from(out);
}

const a0 = read(gl.COLOR_ATTACHMENT0);
const a1 = read(gl.COLOR_ATTACHMENT1);

console.log("ATTACHMENT 0:", a0); // Expect [ 1, 0, 0, 1 ]
console.log("ATTACHMENT 1:", a1); // Bug: Expect [ 0, 1, 0, 1] but returns [ 1, 0, 0, 1]

Changes:

Added a default case in OverrideDrawBufferEnum, to make sure standard WebGL 2 attachment enums are passed through to the driver unchanged.