renderer: unify image.zig

The code in metal/image.zig and opengl/image.zig was virtually identical
save for the texture options, so I've moved that to the GraphicsAPI and
unified them in to renderer/image.zig

pkg/wuffs/src/main.zig

@@ -3,6 +3,7 @@ const std = @import("std");
 pub const png = @import("png.zig");
 pub const jpeg = @import("jpeg.zig");
 pub const swizzle = @import("swizzle.zig");
+pub const Error = @import("error.zig").Error;
 
 pub const ImageData = struct {
     width: u32,