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/Sampler.zig

@@ -0,0 +1,43 @@
+const Sampler = @This();
+
+const std = @import("std");
+const c = @import("c.zig").c;
+const errors = @import("errors.zig");
+const glad = @import("glad.zig");
+const Texture = @import("Texture.zig");
+
+id: c.GLuint,
+
+/// Create a single sampler.
+pub fn create() errors.Error!Sampler {
+    var id: c.GLuint = undefined;
+    glad.context.GenSamplers.?(1, &id);
+    try errors.getError();
+    return .{ .id = id };
+}
+
+/// glBindSampler
+pub fn bind(v: Sampler, index: c_uint) !void {
+    glad.context.BindSampler.?(index, v.id);
+    try errors.getError();
+}
+
+pub fn parameter(
+    self: Sampler,
+    name: Texture.Parameter,
+    value: anytype,
+) errors.Error!void {
+    switch (@TypeOf(value)) {
+        c.GLint => glad.context.SamplerParameteri.?(
+            self.id,
+            @intFromEnum(name),
+            value,
+        ),
+        else => unreachable,
+    }
+    try errors.getError();
+}
+
+pub fn destroy(v: Sampler) void {
+    glad.context.DeleteSamplers.?(1, &v.id);
+}

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");