Improve testing support for `membrane.webgl` backend.

[phronmophobic]

Loading the membrane.webgl namespace will throw an exception if some of the browser APIs are missing. This makes it annoying for writing/running tests outside of the browser environment.

Also, on principle, it probably shouldn't throw an exception when loading the namespace regardless of whether the broswer APIs are available.

[phronmophobic]

Some ideas for suggestions for fixes/improvements:

• Write all the membrane.webgl code against a protocol so that it can be mocked/substituted
• use delays for any top level definitions

[phronmophobic]

Some specific problem expressions:

• membrane.audio also has this problem
• https://github.com/phronmophobic/membrane/blob/7e82b68f7a5b70a5d1c5f7e3db8f91c0230069ad/src/membrane/webgl.cljs#L579C38-L579C38
• https://github.com/phronmophobic/membrane/blob/7e82b68f7a5b70a5d1c5f7e3db8f91c0230069ad/src/membrane/webgl.cljs#L165