Auto merge of #1231 - manuel-rhdt:draw_fb, r=kvark
Auto merge of #1231 - manuel-rhdt:draw_fb, r=kvark

gfx_device_gl: Add a new field allowing to use an externally managed framebuffer object as the display target.

Adds a new public field `display_fb` to the `CommandBuffer` structure in the `gfx_device_gl` backend. If not changed by the user it will have the value 0. This allows to use an existing framebuffer object as the drawing destination instead of the Framebuffer with name 0. Useful on iOS/tvOS where no default frame buffer exists and drawing can only occur on system provided renderbuffers.

I expect this be used with the `create_main_targets_raw` function to create the default color and depth targets. These will automatically use the framebuffer object given in the `display_fb` field.

If you are happy with this I would like to do more pull requests for better iOS support (e.g. only use supported OpenGL ES 3.0 features).

ColinKinloch/gfx,davll/gfx,JohnColanduoni/gfx,ColinKinloch/gfx,JohnColanduoni/gfx,gfx-rs/gfx-rs