jpeg: provide a rough estimate of decoder memory overhead

JPEG decoding requires additional space in the bitmap buffer beyond
what is needed for the decoded pixel data. Provide a way to estimate
how much overhead is needed.

The actual overhead is sizeof(struct jpeg) + decode_buf_size, where
the latter depends on the image size and JPEG encoding used. From my
testing decode_buf_size is normally pretty small (under 5 KiB) but
looking at the code it could be large in some cases, primarily with
large images, so 32 KiB seems to be a decent compromise. Someone who
knows better about JPEG should pick a better value if that's too big.

Using a constant is obviously not the most accurate but it seems to
be the simplest option for retrofitting to existing code.

Change-Id: I573b0abb8ca2d79e43f185010487f07226edb793

apps/recorder/jpeg_load.c

@@ -2228,4 +2228,15 @@ int read_jpeg_fd(int fd,
 }
 #endif
 
+const size_t JPEG_DECODE_OVERHEAD =
+    /* Reserve an arbitrary amount for the decode buffer
+     * FIXME: Somebody who knows what they're doing should look at this */
+    (32 * 1024)
+#ifndef JPEG_FROM_MEM
+    /* Unless the struct jpeg is defined statically, we need to allocate
+     * it in the bitmap buffer as well */
+    + sizeof(struct jpeg)
+#endif
+    ;
+
 /**************** end JPEG code ********************/

apps/recorder/jpeg_load.h

@@ -32,6 +32,13 @@
 #ifndef _JPEG_LOAD_H
 #define _JPEG_LOAD_H
 
+/* Approximate memory overhead required for JPEG decoding. This memory is
+ * taken from the bitmap buffer so you must ensure the buffer is big enough
+ * to contain all decoded pixel data plus decoder overhead, otherwise the
+ * image cannot be loaded. After the image is loaded this extra memory can
+ * be freed. */
+extern const size_t JPEG_DECODE_OVERHEAD;
+
 int read_jpeg_file(const char* filename,
                    struct bitmap *bm,
                    int maxsize,