fix custom-shader writergate breakage (#9061)

Fixes: #9060

src/renderer/shadertoy.zig

@@ -80,9 +80,7 @@ pub fn loadFromFile(
         const file = try cwd.openFile(path, .{});
         defer file.close();
 
-        var buf: [4096]u8 = undefined;
-        var reader = file.reader(&buf);
-        break :src try reader.interface.readAlloc(
+        break :src try file.readToEndAlloc(
             alloc,
             4 * 1024 * 1024, // 4MB
         );