Fix glClear not working on some drivers

[Joao da Silva]

See this TODO:

windowjs/src/window.cc

Line 209 in 2c1cc2b

 // TODO: figure why this is needed on some machines and not others. 

The framebuffer should be cleared with glClear(GL_COLOR_BUFFER_BIT).

However, that doesn't seem to clear anything in an older iMac with a Radean R9 M390 GPU.

This is probably a syncing issue between the GL contexts, or GL state that needs to be reset.