renderer/metal: apply layerContentsPlacement on macOS to fix stretching

Fixes #42

pkg/macos/animation.zig

@@ -0,0 +1,6 @@
+pub const c = @import("animation/c.zig");
+pub usingnamespace @import("animation/layer.zig");
+
+test {
+    @import("std").testing.refAllDecls(@This());
+}

pkg/macos/animation/c.zig

@@ -0,0 +1,3 @@
+pub usingnamespace @cImport({
+    @cInclude("QuartzCore/CALayer.h");
+});

pkg/macos/animation/layer.zig

@@ -0,0 +1,2 @@
+/// https://developer.apple.com/documentation/quartzcore/calayer/contents_gravity_values?language=objc
+pub extern "c" const kCAGravityTopLeft: *anyopaque;

pkg/macos/build.zig

@@ -32,6 +32,7 @@ pub fn build(b: *std.Build) !void {
     lib.linkFramework("CoreGraphics");
     lib.linkFramework("CoreText");
     lib.linkFramework("CoreVideo");
+    lib.linkFramework("QuartzCore");
     if (target.result.os.tag == .macos) {
         lib.linkFramework("Carbon");
         module.linkFramework("Carbon", .{});
@@ -42,6 +43,7 @@ pub fn build(b: *std.Build) !void {
         module.linkFramework("CoreGraphics", .{});
         module.linkFramework("CoreText", .{});
         module.linkFramework("CoreVideo", .{});
+        module.linkFramework("QuartzCore", .{});
 
         if (!target.query.isNative()) {
             try apple_sdk.addPaths(b, &lib.root_module);

pkg/macos/main.zig

@@ -1,4 +1,5 @@
 pub const foundation = @import("foundation.zig");
+pub const animation = @import("animation.zig");
 pub const graphics = @import("graphics.zig");
 pub const os = @import("os.zig");
 pub const text = @import("text.zig");