Fix readPixels PBO segfault & implement getBufferSubData for WebGL2

[Determinated738]

This PR fixes a segfault that happens when using PBOs, as well as implementing getBufferSubData as it was missing.

Minimal code for recreating the segfault:

import createGL from "gl";

const width = 512;
const height = 512;
const size = width * height * 4;

const gl = createGL(width, height, { createWebGL2Context: true });

try {
    const pbo = gl.createBuffer();
    gl.bindBuffer(gl.PIXEL_PACK_BUFFER, pbo);
    gl.bufferData(gl.PIXEL_PACK_BUFFER, size, gl.STREAM_READ);
    gl.bindBuffer(gl.PIXEL_PACK_BUFFER, null);

    gl.clearColor(1, 0, 0, 1);
    gl.clear(gl.COLOR_BUFFER_BIT);

    gl.bindBuffer(gl.PIXEL_PACK_BUFFER, pbo);
    gl.readPixels(0, 0, width, height, gl.RGBA, gl.UNSIGNED_BYTE, 0);

    const pixels = new Uint8Array(size);
    gl.getBufferSubData(gl.PIXEL_PACK_BUFFER, 0, pixels);
    console.log(pixels);
} catch (e) {
    console.error("Caught JS Error:", e);
}