[WebGL] Return only RGBA swapchain formats

[Gordon-F]

WebGL doesn't supports BGRA texture formats

PR checklist:

• [ ] make succeeds (on *nix)
• [ ] make reftests succeeds
• [x] tested examples with the following backends: WebGL

[bors[bot]]

Build succeeded:

• Android Stable
• Dependency Check
• iOS Stable
• MacOS Nightly
• MacOS Stable
• Ubuntu Nightly
• Ubuntu Stable
• Web Assembly
• Windows Nightly
• Windows Stable