Try feeding libpng as many bytes as remain, rather than panicking if …

…there aren't as many as it requested.

imageflow_core/src/codecs/libpng_decoder.rs

@@ -151,9 +151,10 @@ impl PngDec{
 
         let buffer_slice = unsafe{ std::slice::from_raw_parts_mut(buffer, bytes_requested) };
 
-        return match decoder.io.read_exact(buffer_slice) {
-            Ok(()) => {
-                *out_bytes_read = buffer_slice.len();
+        return match decoder.io.read_maximally(buffer_slice) {
+            Ok(read_total) => {
+                assert!(read_total <= bytes_requested);
+                *out_bytes_read = read_total;
                 true
             },
             Err(err) => {

imageflow_core/src/io.rs

@@ -135,6 +135,22 @@ impl IoProxy {
         self.backend.get_read().expect("cannot read from writer").read_exact(buf)
     }
 
+    pub fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize>{
+        self.backend.get_read().expect("cannot read from writer").read(buf)
+    }
+
+    pub fn read_maximally(&mut self, buf: &mut [u8]) -> std::io::Result<usize>{
+        let mut total_read = 0;
+        while total_read < buf.len(){
+            let read = self.read(&mut buf[total_read..])?;
+            if read == 0{
+                break;
+            }
+            total_read += read;
+        }
+        Ok(total_read)
+    }
+
     pub fn read_file(context: &Context, filename: PathBuf, io_id: i32) -> Result<IoProxy> {
         IoProxy::file_with_mode(context,  io_id, filename,IoDirection::In)
     }