Metal backend: compile metal with precise and safe flags on (#16598)

This pull request updates the Metal shader compilation process to
improve numerical accuracy and safety when compiling shaders on newer
versions of macOS and iOS. The main change is the introduction of
specific compile options for math operations when supported by the
operating system.

In the compileLibrary() function, added `MTLCompileOptions` when running
on macOS 15.0, iOS 18.0, or newer, to ensure safer and more precise math
operations during shader compilation:
* `mathMode` set to `MTLMathModeSafe` (equivalent to
-fmetal-math-mode=safe)
* `mathFloatingPointFunctions` set to
`MTLMathFloatingPointFunctionsPrecise` (equivalent to
-fmetal-math-fp32-functions=precise)

Author: manuelcandales

backends/apple/metal/runtime/shims/et_metal.mm

@@ -234,7 +234,13 @@ int metal_copy_memory(void* dst, const void* src, size_t nbytes, bool src_is_dev
         NSString* sourceString = [NSString stringWithUTF8String:shaderSource_.c_str()];
         NSError* error = nil;
 
-        library_ = [device newLibraryWithSource:sourceString options:nil error:&error];
+        MTLCompileOptions* options = [[MTLCompileOptions new] autorelease];
+        if (@available(macOS 15.0, iOS 18.0, tvOS 18.0, watchOS 11.0, *)) {
+            options.mathMode = MTLMathModeSafe;
+            options.mathFloatingPointFunctions = MTLMathFloatingPointFunctionsPrecise;
+        }
+
+        library_ = [device newLibraryWithSource:sourceString options:options error:&error];
         if (!library_ || error) {
             ET_LOG(Error, "ETMetalShaderLibrary: Failed to compile shader library: %s",
                    error ? [[error localizedDescription] UTF8String] : "unknown error");