Add HDR environment map support to GL renderer

[markaren]

• RGBELoader for loading .hdr files
• Equirectangular → cubemap conversion pipeline now wired end-to-end for both background and IBL
• MeshStandardMaterial picks up scene.environment automatically (no material.envMap required)
• 7 pre-existing bugs in the cube render target pipeline fixed as part of the work

Closes #309