lavu: replace int/float punning functions

lavu: replace int/float punning functions

The existing functions defined in intfloat_readwrite.[ch] are
both slow and incorrect (infinities are not handled).

This introduces a new header with fast, inline conversion
functions using direct union punning assuming an IEEE-754
system, an assumption already made throughout the code.

The one use of Intel/Motorola extended 80-bit format is
replaced by simpler code sufficient under the present
constraints (positive normal values).

The old functions are marked deprecated and retained for
compatibility.

Signed-off-by: Mans Rullgard <mans@mansr.com>