all: move standalone libraries to libs/ subdirectory

The root dir of our repository has grown quite a lot the past few months.

I'd like to make it more clear where the bulk of the engine lives (`src/`) and
also make it more clear which Mach libraries are consumable as standalone projects.

As for the name of this directory, `libs` was my first choice but there's a bit of
a convention of that being external libraries in Zig projects _today_, while these
are libraries maintained as part of Mach in this repository - not external ones.

We will name this directory `libs`, and if we have a need for external libraries
we will use `external` or `deps` for that directory name. I considered other names
such as `components`, `systems`, `modules` (which are bad as they overlap with
major ECS / engine concepts), and it seems likely the official Zig package manager
will break the convention of using a `libs` dir anyway.

Performed via:

```sh
mkdir libs/
git mv freetype libs/
git mv basisu libs/
git mv gamemode libs/
git mv glfw libs/
git mv gpu libs/
git mv gpu-dawn libs/
git mv sysaudio libs/
git mv sysjs libs/
git mv ecs libs/
```

git-subtree-dir: glfw
git-subtree-mainline: 0d5b853443
git-subtree-split: 572d1144f11b353abdb64fff828b25a4f0fbb7ca

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

git mv ecs libs/

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

libs/sysaudio/examples/soundio-sine-wave.zig

@@ -0,0 +1,76 @@
+const std = @import("std");
+const soundio = @import("soundio");
+const c = soundio.c;
+const SoundIo = soundio.SoundIo;
+const OutStream = soundio.OutStream;
+
+var seconds_offset: f32 = 0;
+
+fn write_callback(
+    maybe_outstream: ?[*]c.SoundIoOutStream,
+    frame_count_min: c_int,
+    frame_count_max: c_int,
+) callconv(.C) void {
+    _ = frame_count_min;
+    const outstream = OutStream{ .handle = @ptrCast(*c.SoundIoOutStream, maybe_outstream) };
+    const layout = outstream.layout();
+    const float_sample_rate = outstream.sampleRate();
+    const seconds_per_frame = 1.0 / @intToFloat(f32, float_sample_rate);
+    var frames_left = frame_count_max;
+
+    while (frames_left > 0) {
+        var frame_count = frames_left;
+
+        var areas: [*]c.SoundIoChannelArea = undefined;
+        outstream.beginWrite(
+            @ptrCast([*]?[*]c.SoundIoChannelArea, &areas),
+            &frame_count,
+        ) catch |err| std.debug.panic("write failed: {s}", .{@errorName(err)});
+
+        if (frame_count == 0) break;
+
+        const pitch = 440.0;
+        const radians_per_second = pitch * 2.0 * std.math.pi;
+        var frame: c_int = 0;
+        while (frame < frame_count) : (frame += 1) {
+            const sample = std.math.sin((seconds_offset + @intToFloat(f32, frame) *
+                seconds_per_frame) * radians_per_second);
+            {
+                var channel: usize = 0;
+                while (channel < @intCast(usize, layout.channelCount())) : (channel += 1) {
+                    const channel_ptr = areas[channel].ptr;
+                    const sample_ptr = &channel_ptr[@intCast(usize, areas[channel].step * frame)];
+                    @ptrCast(*f32, @alignCast(@alignOf(f32), sample_ptr)).* = sample;
+                }
+            }
+        }
+        seconds_offset += seconds_per_frame * @intToFloat(f32, frame_count);
+        outstream.endWrite() catch |err| std.debug.panic("end write failed: {s}", .{@errorName(err)});
+        frames_left -= frame_count;
+    }
+}
+
+pub fn main() !void {
+    const sio = try SoundIo.init();
+    defer sio.deinit();
+    try sio.connect();
+    sio.flushEvents();
+
+    const default_output_index = sio.defaultOutputDeviceIndex() orelse return error.NoOutputDeviceFound;
+
+    const device = sio.getOutputDevice(default_output_index) orelse return error.OutOfMemory;
+    defer device.unref();
+
+    std.debug.print("Output device: {s}\n", .{device.name()});
+
+    const outstream = try device.createOutStream();
+    defer outstream.deinit();
+
+    outstream.setFormat(.float32LE);
+    outstream.setWriteCallback(write_callback);
+
+    try outstream.open();
+    try outstream.start();
+
+    while (true) sio.waitEvents();
+}