webgl backend uses invalid glow::BGRA constant

[Aaron1011]

Description
On web, attempting to create a texture with TextureFormat::Bgra8Unorm will result in wgpu attempting to call glTexSubImage2D with glow::BGRA:

wgpu/wgpu-hal/src/gles/conv.rs

Line 35 in c51edd3

Tf::Bgra8Unorm => (glow::RGBA8, glow::BGRA, glow::UNSIGNED_BYTE), //TODO?

However, the MDN documents don't list this as a valid format value: https://developer.mozilla.org/en-US/docs/Web/API/WebGLRenderingContext/texSubImage2D

On Chrome, this logs WebGL: INVALID_ENUM: texSubImage2D: invalid format to the console

Repro steps
Create a texture with TextureFormat::Bgra8Unorm on the webgl backend.

Expected vs observed behavior

wgpu should produce a validation error when attempting to create a texture with a format that the backend doesn't support (e.g. Bgra8Unorm on webgl). Currently, no indication is given to Rust code that the texture creation failed.

Extra materials

Platform
wgpu v0.15.1