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/soundio/OutStream.zig

@@ -0,0 +1,58 @@
+const c = @import("c.zig");
+const intToError = @import("error.zig").intToError;
+const Error = @import("error.zig").Error;
+const Format = @import("enums.zig").Format;
+const ChannelLayout = @import("ChannelLayout.zig");
+
+const OutStream = @This();
+
+pub const WriteCallback = if (@import("builtin").zig_backend == .stage1)
+    fn (stream: ?[*]c.SoundIoOutStream, frame_count_min: c_int, frame_count_max: c_int) callconv(.C) void
+else
+    *const fn (stream: ?[*]c.SoundIoOutStream, frame_count_min: c_int, frame_count_max: c_int) callconv(.C) void;
+
+handle: *c.SoundIoOutStream,
+
+pub fn deinit(self: OutStream) void {
+    c.soundio_outstream_destroy(self.handle);
+}
+
+pub fn open(self: OutStream) Error!void {
+    try intToError(c.soundio_outstream_open(self.handle));
+}
+
+pub fn start(self: OutStream) Error!void {
+    try intToError(c.soundio_outstream_start(self.handle));
+}
+
+pub fn beginWrite(self: OutStream, areas: [*]?[*]c.SoundIoChannelArea, frame_count: *i32) Error!void {
+    try intToError(c.soundio_outstream_begin_write(
+        self.handle,
+        areas,
+        frame_count,
+    ));
+}
+
+pub fn endWrite(self: OutStream) Error!void {
+    try intToError(c.soundio_outstream_end_write(self.handle));
+}
+
+pub fn setFormat(self: OutStream, format: Format) void {
+    self.handle.*.format = @enumToInt(format);
+}
+
+pub fn setWriteCallback(self: OutStream, callback: WriteCallback) void {
+    self.handle.*.write_callback = callback;
+}
+
+pub fn layout(self: OutStream) ChannelLayout {
+    return ChannelLayout{ .handle = self.handle.*.layout };
+}
+
+pub fn sampleRate(self: OutStream) i32 {
+    return self.handle.*.sample_rate;
+}
+
+pub fn layoutError(self: OutStream) Error!void {
+    try intToError(self.handle.*.layout_error);
+}