Adding the ability to access MODULATE in the shader breaks when final_modulate is baked into vertex colors (this is a technique used to batch together different colored items). This PR prevents baking vertex colors when MODULATE is detected in the shader.
It also prevents baking when COLOR is read in canvas shaders, which could currently produce the wrong result in the shader if colors were baked. It does not prevent baking if COLOR is only written, which happens in most shaders, and will operate correctly without baking.
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