Add alpha channel if RenderFlags.RGBA is set

[eyllanesc]

Fixes #209

[alon-ne]

Hey Edwin, I also ran into the issue that I get RGB instead of RGBA, specifically when I use osmesa on mac (running with the normal graphics adaptor I got RGBA as expected).
I validated that running on linux with osmesa, the issue is indeed resolved, thank you!

On mac with osmesa however, I tried using this branch and it indeed produced an RGBA image, but it's cut so I see only the lower-left quarter. Here's a snippet of my code

            scene = pyrender.Scene.from_trimesh_scene(self.glb, bg_color=[0] * 4, ambient_light=[255] * 3)
            scene.add(self._camera(), pose=self.glb.camera_transform)

            renderer = pyrender.OffscreenRenderer(width, height)
            thumbnail, _ = renderer.render(scene, RenderFlags.RGBA)
            thumbnail_filename = self._thumbnail_filename(destination)
            Image.fromarray(thumbnail).save(thumbnail_filename)

[alon-ne]

There's another issue about this, specifically with osmesa
#137

[alon-ne]

Attaching an example image produced by this code, when run with PYOPENGL_PLATFORM=osmesa it's cut, but without the environment variable it's not cut.

[jkulhanek]

Any progress here?