Error check using IsValid functions and return error unions (#191)

* Verify shaders are valid

* Verify images are valid

* Verify models are valid

* Verify materials are valid

* Verify textures are valid

* Verify render textures are valid

* Verify waves are valid

* Verify sounds are valid

* Verify music is valid

* Verify audio streams are valid

* Verify fonts are valid

* Update examples to handle error unions

examples/audio/raw_stream.zig

@@ -41,7 +41,7 @@ pub fn main() anyerror!void {
     rl.setAudioStreamBufferSizeDefault(MAX_SAMPLES_PER_UPDATE);
 
     // Init raw audio stream (sample rate: 44100, sample size: 16bit-short, channels: 1-mono)
-    const stream = rl.loadAudioStream(44100, 16, 1);
+    const stream = try rl.loadAudioStream(44100, 16, 1);
     defer rl.unloadAudioStream(stream); // Close raw audio stream and delete buffers from RAM
 
     rl.setAudioStreamCallback(stream, &audioInputCallback);