Fix #309665: Alpha (transparency) not showing on several symbols and …

[elerouxx]

…elements

Resolves: https://musescore.org/en/node/309665

Fixes Alpha channel (transparency) not honored in the drawing of Symbols.
Just added a std::min() function in the symbol-to-bitmap converter in the symbol Draw function, so the bitmap pixels' alpha value is never higher than the alpha value set by the user.

• I signed CLA
• I made sure the code in the PR follows the coding rules
• I made sure the code compiles on my machine
• I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"
• I made sure the commit message title starts with "fix #424242:" if there is a related issue
• [N/A] I created the test (mtest, vtest, script test) to verify the changes I made