Add zero-allocation texture encoding support

[Cosmin-B]

Summary

This PR adds support for encoding textures to various image formats (PNG, JPG, TGA, EXR) with zero managed memory allocations by exposing Unity's internal encoding methods.

Features Added

• BlittableArrayWrapper: Wraps Unity's native memory for encoded image data
• NativeArray-based encoding: Returns NativeArray<byte> for full control over memory lifetime
• Span-based encoding: Returns Span<byte> for temporary access to encoded data
• Multiple formats supported: PNG, JPG (with quality), TGA, and EXR (with HDR flags)

Implementation Details

• Extended UnityExposed with encoding method stubs (EncodeToPNG_Injected, etc.)
• Created ImageEncodingExtensions with user-friendly extension methods
• Updated Cecil processor to handle BlittableArrayWrapper conversion between Unity's internal type and our exposed type
• Added comprehensive test suite (TestTextureEncoding.cs) covering all formats

Performance Benefits

• Zero allocations: No managed byte[] arrays created during encoding
• Direct native memory access: Work directly with Unity's encoded data
• GC pressure reduction: Especially beneficial for frequent encoding operations
• Job System compatible: NativeArray results can be used in Unity Jobs

Testing

Tested successfully on:

• Unity Editor (Mono)
• IL2CPP builds (Development and Release)
• All supported platforms

Usage Example

// Encode to NativeArray (caller manages lifetime)
using (NativeArray<byte> pngData = texture.EncodeToPNGNative(Allocator.Temp))
{
    // Use encoded data...
}

// Encode to Span (temporary access)
Span<byte> jpgData = texture.EncodeToJPGSpan(quality: 90);

Compatibility

• Unity 6000.0.31f1+
• Full IL2CPP support
• All platforms supported

Closes #[issue_number] (if applicable)