Unable to clear color buffers other than 0

`ClearStage` uses `RenderInterface::obtainFBO(size_t index, AbstractRenderTarget * renderTarget)` to get an FBO for a specific attachment. This overload does not set draw buffers and neither does the clear stage. This works for color buffer 0, as the default value for the draw buffers array is [GL_COLOR_ATTACHMENT0, GL_NONE, GL_NONE, ...] (see https://computergraphics.stackexchange.com/a/3937), but fails for any additional color buffer.

Who should be responsible for seting the draw buffers?

Start with ClearStage and RenderInterface::obtainFBO(size_t index, AbstractRenderTarget * renderTarget), then trace how draw buffers are configured for framebuffer attachments. Done means establishing the responsibility for setting draw buffers and ensuring clearing works for color attachments beyond index 0.

Written by the indexing model from the issue text.