Add qoi_encode_to_buf() and encode_size_required()

src/error.rs

@@ -10,6 +10,7 @@ pub enum Error {
     EmptyImage { width: u32, height: u32 },
     BadEncodingDataSize { size: usize, expected: usize },
     BadDecodingDataSize { size: usize },
+    OutputBufferTooSmall { size: usize, required: usize },
     InvalidMagic { magic: u32 },
     // TODO: invalid colorspace
 }
@@ -32,6 +33,9 @@ impl Display for Error {
                 let min_size = QOI_HEADER_SIZE + QOI_PADDING;
                 write!(f, "bad data size when decoding: {} (minimum required: {})", size, min_size)
             }
+            Self::OutputBufferTooSmall { size, required } => {
+                write!(f, "output buffer size too small: {} (minimum required: {})", size, required)
+            }
             Self::InvalidMagic { magic } => {
                 write!(f, "invalid magic: expected {:?}, got {:?}", QOI_MAGIC, magic)
             }