Fixes a bug with the debug_printf and debug_printfln macros not e…

…scaping '{' and '}' correctly.

The `debug_printf` and `debug_printfln` passes its format string to the `asm!` macro, which uses `{` and `}` to separate its arguments. Passing `{` and `}` to `asm!` directly is incorrect, and leads to compilation issues:
```
error: invalid asm template string: expected `'}'`, found `'"'`
     --> examples/shaders/sky-shader/src/lib.rs:13016:2
      |
13015 | unsafe{debug_printf!("Variant3{")};
      |        -------------------------- in this macro invocation
13016 |         unsafe{debug_printf!("fld0:")};
      | ----^ expected `'}'` in asm template string
      | |
      | because of this opening brace
      |
      = note: if you intended to print `{`, you can escape it using `{{`
      = note: this error originates in the macro `debug_printf` (in Nightly builds, run with -Z macro-backtrace for more info)
``` 

This commit escapes those characters using `{{` and `}}`, which removes this issue and produces correct behaviour.

crates/spirv-std/macros/src/lib.rs

@@ -613,7 +613,11 @@ fn debug_printf_inner(input: DebugPrintfInput) -> TokenStream {
         .into_iter()
         .collect::<proc_macro2::TokenStream>();
     let op_loads = op_loads.into_iter().collect::<proc_macro2::TokenStream>();
-
+    // Escapes the '{' and '}' characters in the format string. 
+    // Since the `asm!` macro expects '{' '}' to surround its arguments, we have to use '{{' and '}}' instead.
+    // The `asm!` macro will then later turn them back into '{' and '}'.
+    let format_string = format_string.replace('{',"{{").replace('}',"}}");
+
     let op_string = format!("%string = OpString {format_string:?}");
 
     let output = quote::quote! {