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.