renderer: create explicit sampler state for custom shaders

The GLSL to MSL conversion process uses a passed-in sampler state for
the `iChannel0` parameter and we weren't providing it. This magically
worked on Apple Silicon for unknown reasons but failed on Intel GPUs.

In normal, hand-written MSL, we'd explicitly create the sampler state as
a normal variable (we do this in `shaders.metal` already!), but the
Shadertoy conversion stuff doesn't do this, probably because the exact
sampler parameters can't be safely known.

This fixes a Metal validation error when using custom shaders:

```
-[MTLDebugRenderCommandEncoder validateCommonDrawErrors:]:5970: failed 
assertion `Draw Errors Validation Fragment Function(main0): missing Sampler 
binding at index 0 for iChannel0Smplr[0].
```

pkg/opengl/main.zig

@@ -18,6 +18,7 @@ pub const Buffer = @import("Buffer.zig");
 pub const Framebuffer = @import("Framebuffer.zig");
 pub const Renderbuffer = @import("Renderbuffer.zig");
 pub const Program = @import("Program.zig");
+pub const Sampler = @import("Sampler.zig");
 pub const Shader = @import("Shader.zig");
 pub const Texture = @import("Texture.zig");
 pub const VertexArray = @import("VertexArray.zig");