src/gpu: move github.com/hexops/mach-gpu here

This moves github.com/hexops/mach-gpu@528dad0823dafeae5d474c88cc658b091bf2e605 to
this repository in the src/gpu directory. It can be imported via `@import("mach").gpu`.

Soon we will move away from mach-gpu entirely as part of #1166 - but in the meantime
I am giving a workshop at https://sycl.it and it would be nice for people using the
`mach.gpu.*` API to be able to search the API in this single repository.

There's not much harm to moving this code here.

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

src/gpu/external_texture.zig

@@ -0,0 +1,56 @@
+const Bool32 = @import("main.zig").Bool32;
+const ChainedStruct = @import("main.zig").ChainedStruct;
+const TextureView = @import("texture_view.zig").TextureView;
+const Origin2D = @import("main.zig").Origin2D;
+const Extent2D = @import("main.zig").Extent2D;
+const Impl = @import("interface.zig").Impl;
+
+pub const ExternalTexture = opaque {
+    pub const BindingEntry = extern struct {
+        chain: ChainedStruct = .{ .next = null, .s_type = .external_texture_binding_entry },
+        external_texture: *ExternalTexture,
+    };
+
+    pub const BindingLayout = extern struct {
+        chain: ChainedStruct = .{ .next = null, .s_type = .external_texture_binding_layout },
+    };
+
+    const Rotation = enum(u32) {
+        rotate_0_degrees = 0x00000000,
+        rotate_90_degrees = 0x00000001,
+        rotate_180_degrees = 0x00000002,
+        rotate_270_degrees = 0x00000003,
+    };
+
+    pub const Descriptor = extern struct {
+        next_in_chain: ?*const ChainedStruct = null,
+        label: ?[*:0]const u8 = null,
+        plane0: *TextureView,
+        plane1: ?*TextureView = null,
+        visible_origin: Origin2D,
+        visible_size: Extent2D,
+        do_yuv_to_rgb_conversion_only: Bool32 = .false,
+        yuv_to_rgb_conversion_matrix: ?*const [12]f32 = null,
+        src_transform_function_parameters: *const [7]f32,
+        dst_transform_function_parameters: *const [7]f32,
+        gamut_conversion_matrix: *const [9]f32,
+        flip_y: Bool32,
+        rotation: Rotation,
+    };
+
+    pub inline fn destroy(external_texture: *ExternalTexture) void {
+        Impl.externalTextureDestroy(external_texture);
+    }
+
+    pub inline fn setLabel(external_texture: *ExternalTexture, label: [*:0]const u8) void {
+        Impl.externalTextureSetLabel(external_texture, label);
+    }
+
+    pub inline fn reference(external_texture: *ExternalTexture) void {
+        Impl.externalTextureReference(external_texture);
+    }
+
+    pub inline fn release(external_texture: *ExternalTexture) void {
+        Impl.externalTextureRelease(external_texture);
+    }
+};