Add multipass pass support (buffers A-D)

CHANGELOG.md

@@ -22,6 +22,7 @@ Possible sections in each release:
 Added:
 * Run shaders from the website API https://github.com/pygfx/shadertoy/pull/25
 * Additional Uniforms are now supported in `.screenshot()` https://github.com/pygfx/shadertoy/pull/37
+* Multipass shaders using `BufferRenderPass` and `ShadertoyChannelBuffer` https://github.com/pygfx/shadertoy/pull/43
 
 Changed:
 * `ShadertoyChannel` is now more specific `ShadertoyChannelTexture` https://github.com/pygfx/shadertoy/pull/40

README.md

@@ -66,6 +66,7 @@ shader = Shadertoy.from_id("NslGRN")
 ```
 
 When passing `off_screen=True` the `.snapshot()` method allows you to render individual frames with chosen uniforms.
+Be aware that based on your device and backend, the preferred format might be BRGA, so the channels need to be swapped to get an RGBA image.
 ```python
 shader = Shadertoy(shader_code, resolution=(800, 450), off_screen=True)
 frame0_data = shader.snapshot()