Rename raw input to raw mouse motion, cleanup

This renames 'raw input' to 'raw mouse motion' as there are other kinds
of raw input.  The update path is restructured to avoid reinitializing
all of disabled cursor mode.  Modification of shared state is moved out
into shared code.  Raw mouse motion is disabled by default for
compatibility.

Related to #1401.

docs/input.dox

@@ -233,7 +233,7 @@ arguments can always be passed unmodified to this function.
 
 @section input_mouse Mouse input
 
-Mouse input comes in many forms, including cursor motion, button presses and
+Mouse input comes in many forms, including mouse motion, button presses and
 scrolling offsets.  The cursor appearance can also be changed, either to
 a custom image or a standard cursor shape from the system theme.
 
@@ -308,6 +308,31 @@ glfwSetInputMode(window, GLFW_CURSOR, GLFW_CURSOR_NORMAL);
 @endcode
 
 
+@anchor GLFW_RAW_MOUSE_MOTION
+@subsection raw_mouse_motion Raw mouse motion
+
+When the cursor is disabled, raw (unscaled and unaccelerated) mouse motion can
+be enabled if available.
+
+Raw mouse motion is closer to the actual motion of the mouse across a surface.
+It is not affected by the scaling and acceleration applied to the motion of the
+desktop cursor.  That processing is suitable for a cursor while raw motion is
+better for controlling for example a 3D camera.  Because of this, raw mouse
+motion is only provided when the cursor is disabled.
+
+Call @ref glfwRawMouseMotionSupported to check if the current machine provides
+raw motion and set the `GLFW_RAW_MOUSE_MOTION` input mode to enable it.  It is
+disabled by default.
+
+@code
+if (glfwRawMouseMotionSupported())
+    glfwSetInputMode(window, GLFW_RAW_MOUSE_MOTION, GLFW_TRUE);
+@endcode
+
+If supported, raw mouse motion can be enabled or disabled per-window and at any
+time but it will only be provided when the cursor is disabled.
+
+
 @subsection cursor_object Cursor objects
 
 GLFW supports creating both custom and system theme cursor images, encapsulated