webp handler: improve quality-level handling

In lossless mode, libwebp interpretes the quality setting as a
compression-effort setting instead. The code used to set it to 100
(maximum), which could lead to unreasonable compression times (several
seconds for a small image). Instead set it to libweb's own default value.

Also, since the alpha channel compression has its own quality setting,
make sure it follows the main quality/effort setting.

Change-Id: I595d42c61f61c9932b27b185745bde6e7cfb3526
Reviewed-by: Liang Qi <liang.qi@qt.io>