Fix sRGB support for WGL and EGL

[derezzedex]

This PR fixes sRGB support for the wgl and egl backends (which I encountered while trying sRGB with OpenGL ES).

• [x] Tested on all platforms changed
• [x] Compilation warnings were addressed
• [ ] cargo fmt has been run on this branch
• [x] cargo doc builds successfully
• [ ] Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• [ ] Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• [ ] Created or updated an example program if it would help users understand this functionality