Fix a buffer overflow in `png_init_read_transformations`

The palette compositing code in `png_init_read_transformations` was
incorrectly applying background compositing when PNG_FLAG_OPTIMIZE_ALPHA
was set. This violated the premultiplied alpha invariant
`component <= alpha` expected by `png_image_read_composite`, causing
values that exceeded the valid range for the PNG_sRGB_FROM_LINEAR lookup
tables.

When PNG_ALPHA_OPTIMIZED is active, palette entries should contain pure
premultiplied RGB values without background compositing. The background
compositing must happen later in `png_image_read_composite` where the
actual background color from the PNG file is available.

The fix consists in introducing conditional behavior based on
PNG_FLAG_OPTIMIZE_ALPHA: when set, the code performs only
premultiplication using the formula `component * alpha + 127) / 255`
with proper gamma correction. When not set, the original background
compositing calculation based on the `png_composite` macro is preserved.

This prevents buffer overflows in `png_image_read_composite` where
out-of-range premultiplied values would cause out-of-bounds array access
in `png_sRGB_base[]` and `png_sRGB_delta[]`.

Reported-by: Samsung-PENTEST <Samsung-PENTEST@users.noreply.github.com>
Analyzed-by: John Bowler <jbowler@acm.org>