Add cursor mode GLFW_CURSOR_CAPTURED

This adds a cursor mode that provides a visible cursor confined to the
content area of the window.

Fixes #58

docs/input.dox

@@ -312,6 +312,16 @@ glfwSetInputMode(window, GLFW_CURSOR, GLFW_CURSOR_HIDDEN);
 
 This mode puts no limit on the motion of the cursor.
 
+If you wish the cursor to be visible but confined to the content area of the
+window, set the cursor mode to `GLFW_CURSOR_CAPTURED`.
+
+@code
+glfwSetInputMode(window, GLFW_CURSOR, GLFW_CURSOR_CAPTURED);
+@endcode
+
+The cursor will behave normally inside the content area but will not be able to
+leave unless the window loses focus.
+
 To exit out of either of these special modes, restore the `GLFW_CURSOR_NORMAL`
 cursor mode.
 
@@ -319,6 +329,8 @@ cursor mode.
 glfwSetInputMode(window, GLFW_CURSOR, GLFW_CURSOR_NORMAL);
 @endcode
 
+If the cursor was disabled, this will move it back to its last visible position.
+
 
 @anchor GLFW_RAW_MOUSE_MOTION
 @subsection raw_mouse_motion Raw mouse motion

docs/news.dox

@@ -58,6 +58,14 @@ requesting a specific rendering backend when using
 contexts.
 
 
+@subsubsection captured_cursor_34 Captured cursor mode
+
+GLFW now supports confining the cursor to the window content area with the @ref
+GLFW_CURSOR_CAPTURED cursor mode.
+
+For more information see @ref cursor_mode.
+
+
 @subsubsection features_34_init_allocator Support for custom memory allocator
 
 GLFW now supports plugging a custom memory allocator at initialization with @ref
@@ -234,6 +242,7 @@ then GLFW will fail to initialize.
  - @ref GLFW_ANGLE_PLATFORM_TYPE_VULKAN
  - @ref GLFW_ANGLE_PLATFORM_TYPE_METAL
  - @ref GLFW_X11_XCB_VULKAN_SURFACE
+ - @ref GLFW_CURSOR_CAPTURED
 
 
 @section news_archive Release notes for earlier versions