Replace an old workaround for alpha-only shadows

This removes the Indexed8 format hack mentioned in !1225 (merged). This was used for gray/alpha-only shadows in the OpenGL scene; under my config, these are used for dropdown lists for menus/comboboxes. (A nice way to check that KWin uses this code path is to adjust the shadow texture swizzling at scene_opengl.cpp#L1798 and turn affected shadows red.)

Technically this a breaking change for libkwineffects, but not a very significant one: I was not able to find any other code using GLTexture and also using the Indexed8 format hack. (I checked via Debian Code Search, Github's code search, and looked at the few scripts/effects in https://store.kde.org/browse?cat=349&ord=latest which seemed likely to use GLTexture; any that did used only used ARGB formats.)