message: use existing gl context for capability testing
  fixes unit-test
diff: -import { isDebugMode } from '../../mol-util/debug';
  /**
  * Copyright (c) 2018-2021 mol* contributors, licensed under MIT, See LICENSE file for more info.
  *
  * @author Alexander Rose <alexander.rose@weirdbyte.de>
  */
  -import { getErrorDescription, getGLContext } from './context';
  +import { isDebugMode } from '../../mol-util/debug';
  +import { getErrorDescription } from './context';
  import { getProgram } from './program';
  import { getShader } from './shader';
  @@ -167,7 +167,8 @@ export function getColorBufferFloat(gl: GLRenderingContext): COMPAT_color_buffer
  const ext = gl.getExtension('WEBGL_color_buffer_float');
  if (ext === null) {
  // test as support may not be advertised by browsers
  - return testColorBuffer(gl.FLOAT, 'OES_texture_float') ? { RGBA32F: 0x8814 } : null;
  + gl.getExtension('OES_texture_float');
  + return testColorBuffer(gl, gl.FLOAT) ? { RGBA32F: 0x8814 } : null;
  }
  gl.getExtension('EXT_float_blend');
  return { RGBA32F: ext.RGBA32F_EXT };
  @@ -187,7 +188,8 @@ export function getColorBufferHalfFloat(gl: GLRenderingContext): COMPAT_color_bu
  const ext = gl.getExtension('EXT_color_buffer_half_float');
  if (ext === null) {
  // test as support may not be advertised by browsers
  - return testColorBuffer(0x8D61, 'OES_texture_half_float') ? { RGBA16F: 0x881A } : null;
  + gl.getExtension('OES_texture_half_float');
  + return testColorBuffer(gl, 0x8D61) ? { RGBA16F: 0x881A } : null;
  }
  gl.getExtension('EXT_float_blend');
  return { RGBA16F: ext.RGBA16F_EXT };
  @@ -340,20 +342,8 @@ const TextureTestTexCoords = new Float32Array([
  -1.0, -1.0, 1.0, -1.0, -1.0, 1.0, -1.0, 1.0, 1.0, -1.0, 1.0, 1.0
  ]);
  -export function testColorBuffer(type: number, ext: string) {
  // adapted from https://stackoverflow.com/questions/28827511/
  -
  - // Get A WebGL context
  - const canvas = document.createElement('canvas');
  - canvas.width = 16;
  - canvas.height = 16;
  - canvas.style.width = `${16}px`;
  - canvas.style.height = `${16}px`;
  - const gl = getGLContext(canvas);
  - if (gl === null) throw new Error('Unable to get WebGL context');
  -
  - gl.getExtension(ext);
  -
  +export function testColorBuffer(gl: GLRenderingContext, type: number) {
  // setup shaders
  const vertShader = getShader(gl, { type: 'vert', source: TextureTestVertShader });
  const fragShader = getShader(gl, { type: 'frag', source: TextureTestFragShader });
  