Support ARB_framebuffer_sRGB.

Hell, this extension (and EXT_sRGB on ES) is written in so complicated
language that I'm not sure at all if I understood it right and
implemented completely.

doc/opengl-support.dox

@@ -61,7 +61,7 @@ GLSL 1.30                                   | done
 @extension{ARB,texture_rg}                  | done
 @extension{ARB,vertex_array_object}         | done
 @extension{ARB,framebuffer_object}          | done
-@extension{ARB,framebuffer_sRGB}            | |
+@extension{ARB,framebuffer_sRGB}            | done
 @extension{ARB,half_float_vertex}           | done
 @extension{EXT,gpu_shader4}                 | done
 @extension{EXT,packed_float}                | done

src/Magnum/Renderer.h

@@ -135,6 +135,14 @@ class MAGNUM_EXPORT Renderer {
             FaceCulling = GL_CULL_FACE,
 
             #ifndef MAGNUM_TARGET_GLES
+            /**
+             * sRGB encoding of the default framebuffer
+             * @requires_gl30 %Extension @extension{ARB,framebuffer_sRGB}
+             * @requires_gl sRGB encoding of the default framebuffer is
+             *      implementation-defined in OpenGL ES.
+             */
+            FramebufferSRGB = GL_FRAMEBUFFER_SRGB,
+
             /**
              * Logical operation
              * @see @ref setLogicOperation()