Fix #93 in which HDR brightness settings were only partially applied.

When KWin is running in HDR mode, decorations within the compositor must explicitly apply color management when doing their own low-level rendering outside of Qt and KDE APIs. In this case, full intensity is interpreted as the most intense light that the display is capable of outputting. Two smod decorations (aeroglassblur and smodglow) used OpenGL contexts without explicit color management to respect KDE-specified brightness settings when operating in HDR mode. Additionally, these decorations had incorrect brightness scaling code that only applied brightness changes to R, G, and B, but not the Alpha component, resulting in dark glows at low brightness values.

This change fixes the brightness scaling code and introduces explicit brightness scaling in HDR windows that use OpenGL effects to respect the global KDE brightness settings.