implement explicit test for rendering to float textures

Especially for WebGL1 where `OES_texture_float` is not fully indicative if rendering to float textures is possible. `WEBGL_color_buffer_float` should tell us if it is possible, but even when it is, the extension is often not reported by browsers.

Start by locating the WebGL1 capability checks involving OES_texture_float and WEBGL_color_buffer_float. Add an explicit rendering test for float textures, and verify that it detects whether rendering is possible even when the browser does not report WEBGL_color_buffer_float.

Written by the indexing model from the issue text.