Shader Source Transitions

[Xaymar]

Explain the Feature/Enhancement

OBS Studio 27.x added Source transitions, which are not supported by Shaders at the moment. In order to fully support 27.x of OBS Studio, Source transitions must be supported by Shaders.

What makes this feature/enhancement useful for StreamFX and its users?

Fixes #578.

Checklist

• I am offering a monetary bounty/reward for the implementation of this Feature/Enhancement.
• There is no other way to currently do this in StreamFX or OBS Studio.

Any additional Information we need to know?

Shaders currently use the canvas size if they weren't assigned a fixed size, which was correct in the past. Shaders should instead use the transition size in the future, which is given by libOBS.