Expose EXT_color_buffer_half_float if RGBA16F is renderable. The EXT_color_buffer_half_float extension spec issue #2 states that none of the added color buffer formats are required to be renderable and the client must check for framebuffer completeness for using them. Chrome exposes the extension if at least RGBA16F is renderable for WebGL. Replicate Chrome's behaviour by loosening our requirements to only expose this extension if an explicit check for RGBA16F renderability succeeds. BUG=angleproject:2984 Change-Id: Id97f3043ebf3fd11b5e9e2d505e57b76baba9716 Reviewed-on: https://chromium-review.googlesource.com/c/1351350 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>