shaders/sampling: generalize PL_SHADER_SIG_SAMPLER2D
This allows us to accept sampler types other than sampler2D. In principle, any sampler type is supported, but due to shaders/sampling being the only user of this API, we can restrict ourselves to only 2D samplers in the implementation. Notably, though, we also support e.g. PL_SAMPLER_RECT.
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