GL: implement EXT_depth_clamp on WebGL.

doc/opengl-support.dox

@@ -574,7 +574,7 @@ Extension                                   | Status
 @webgl_extension{EXT,texture_norm16}        | done
 @webgl_extension{EXT,polygon_offset_clamp}  | missing support in Emscripten
 @webgl_extension{EXT,clip_control}          | missing support in Emscripten
-@webgl_extension{EXT,depth_clamp}           | |
+@webgl_extension{EXT,depth_clamp}           | done
 @webgl_extension{EXT,texture_mirror_clamp_to_edge} | |
 @webgl_extension{KHR,parallel_shader_compile} | done
 @webgl_extension{NV,shader_noperspective_interpolation} | done (shading language only)

src/Magnum/GL/Renderer.h

@@ -332,19 +332,17 @@ class MAGNUM_GL_EXPORT Renderer {
             #endif
             #endif
 
-            #ifndef MAGNUM_TARGET_WEBGL
             /**
              * Depth clamping. If enabled, ignores near and far clipping plane.
              * @requires_gl32 Extension @gl_extension{ARB,depth_clamp}
              * @requires_es_extension Extension @gl_extension{EXT,depth_clamp}
-             * @requires_gles Depth clamping is not available in WebGL.
+             * @requires_webgl_extension Extension @webgl_extension{EXT,depth_clamp}
              */
             #ifndef MAGNUM_TARGET_GLES
             DepthClamp = GL_DEPTH_CLAMP,
             #else
             DepthClamp = GL_DEPTH_CLAMP_EXT,
             #endif
-            #endif
 
             /**
              * Depth test