libobs/graphics: Add ability to set shader texture sampler

(Note: This also modifies libobs-d3d11 and libobs-opengl)

Allows overriding the sampler for a specific shader parameter.

libobs-d3d11/d3d11-subsystem.hpp

@@ -356,6 +356,7 @@ struct gs_shader_param {
 	gs_shader_param_type           type;
 
 	uint32_t                       textureID;
+	struct gs_sampler_state        *nextSampler = nullptr;
 
 	int                            arrayCount;