Update to latest Zig master (0.10.0-dev.2017+a0a2ce92c) (#261)

* use `@ceil` instead of `std.math.ceil`
* `ChildProcess.init` does not allocate anymore
* update CI zig version
* examples: temporarily switch to fork of zigimg compatible with zig-master

Commands executed:

```
git submodule set-url -- examples/libs/zigimg https://github.com/slimsag/zigimg
git submodule set-branch --branch zig-master -- examples/libs/zigimg
git submodule update --init --remote examples/libs/zigimg
```

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

glfw/system_sdk.zig

@@ -234,7 +234,7 @@ fn determineSdkRoot(allocator: std.mem.Allocator, org: []const u8, name: []const
 }
 
 fn exec(allocator: std.mem.Allocator, argv: []const []const u8, cwd: []const u8) !void {
-    const child = try std.ChildProcess.init(argv, allocator);
+    var child = std.ChildProcess.init(argv, allocator);
     child.cwd = cwd;
     _ = try child.spawnAndWait();
 }