"float alpha()" is potentially confusing, and should be "float opacity()"

"float alpha()" is potentially confusing, and should be "float opacity()".

This is because there are two aspects to alpha blending of a surface:
  1. The surface's own alpha channel, which can have different values for each pixel (hence making surfaces of any shape and transparent in places).
  2. The overall surface opacity value desired by the shell/compositor.

In Mir we presently mean #2 when using "float alpha()". But I think it should be renamed to "float opacity()" to minimize confusion in future with #1.