avutil: Add functions for allocating opaque contexts for algorithms

The current API where the plain size is exposed is not of much
use - in most cases it is allocated dynamically anyway.

If allocated e.g. on the stack via an uint8_t array, there's no
guarantee that the struct's members are aligned properly (unless
the array is overallocated and the opaque pointer within it
manually aligned to some unspecified alignment).

Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/aes.h

@@ -23,16 +23,26 @@
 
 #include <stdint.h>
 
+#include "attributes.h"
+#include "version.h"
+
 /**
  * @defgroup lavu_aes AES
  * @ingroup lavu_crypto
  * @{
  */
 
-extern const int av_aes_size;
+#if FF_API_CONTEXT_SIZE
+extern attribute_deprecated const int av_aes_size;
+#endif
 
 struct AVAES;
 
+/**
+ * Allocate an AVAES context.
+ */
+struct AVAES *av_aes_alloc(void);
+
 /**
  * Initialize an AVAES context.
  * @param key_bits 128, 192 or 256