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

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