Load winsys and gl dynamically in the windows plugin

The dynamic builds (-opengl dynamic) are now functional on Windows.

In such a build no components in Qt link to any OpenGL libraries directly
and qmake will not automatically add any such libraries to the
applications' makefiles. Instead, the libraries are chosen and loaded
during runtime and applications are expected to use QOpenGLFunctions
instead of direct OpenGLfunction calls.

Set the environment variable QT_OPENGL to desktop or angle to skip testing
and force the given implementation. The application attributes (AA_UseOpenGLES
and such) are also taken into account.

The testing logic is same as before: We try to load opengl32 and
resolve a shader related function. If this fails, ANGLE is chosen. This
allows utilizing full desktop OpenGL on systems that have proper drivers,
while a transparent fallback to ANGLE will be done automatically for
systems that don't. The latter includes also remote desktop connections.

Software rendering via Mesa llvmpipe is supported too. The fallback is
automatic on systems where the desktop test fails and ANGLE fails to load
or initialize (e.g. due to missing libs like d3dcompiler), as long as a
suitable patched build of Mesa is available.

[ChangeLog][QtGui] Dynamic OpenGL implementation loading is now supported
on Windows. This requires Qt to be configured with -opengl dynamic.

Task-number: QTBUG-36483
Change-Id: Ie8bb25a6d55b3a1609b00150aeccd909aec27313
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

2 files changed, 17 insertions, 4 deletions

diff --git a/src/corelib/global/qnamespace.h b/src/corelib/global/qnamespace.h
index 839d352d36..3ec859dbb7 100644
--- a/src/corelib/global/qnamespace.h
+++ b/src/corelib/global/qnamespace.h
@@ -521,6 +521,7 @@ public:
         AA_ForceRasterWidgets = 14,
         AA_UseDesktopOpenGL = 15,
         AA_UseOpenGLES = 16,
+        AA_UseSoftwareOpenGL = 17,
 
         // Add new attributes before this line
         AA_AttributeCount
diff --git a/src/corelib/global/qnamespace.qdoc b/src/corelib/global/qnamespace.qdoc
index 8ade3f86d6..873c1bf0cc 100644
--- a/src/corelib/global/qnamespace.qdoc
+++ b/src/corelib/global/qnamespace.qdoc
@@ -168,14 +168,26 @@
     \value AA_ForceRasterWidgets Make top-level widgets use pure raster surfaces,
            and do not support non-native GL-based child widgets.
 
-    \value AA_UseDesktopOpenGL Forces the usage of the desktop OpenGL on
-           platforms that use dynamic loading of the OpenGL implementation.
+    \value AA_UseDesktopOpenGL Forces the usage of desktop OpenGL (for example,
+           \e opengl32.dll or \e libGL.so) on platforms that use dynamic loading
+           of the OpenGL implementation.
            This value has been added in Qt 5.3.
 
-    \value AA_UseOpenGLES Forces the usage of OpenGL ES 2.0 on platforms that
-           use dynamic loading of the OpenGL implementation.
+    \value AA_UseOpenGLES Forces the usage of OpenGL ES 2.0 or higher on
+           platforms that use dynamic loading of the OpenGL implementation.
            This value has been added in Qt 5.3.
 
+    \value AA_UseSoftwareOpenGL Forces the usage of a software based OpenGL
+           implementation on platforms that use dynamic loading of the OpenGL
+           implementation. This will typically be a patched build of
+           \l{http://www.mesa3d.org/llvmpipe.html}{Mesa llvmpipe}, providing
+           OpenGL 2.1. The value may have no effect if no such OpenGL
+           implementation is available. The default name of this library is
+           QtSoftwareOpenGL.dll and can be overridden by setting the environment
+           variable \e QT_OPENGL_DLL. See the platform-specific pages, for
+           instance \l{Qt for Windows}, for more information.  This value has
+           been added in Qt 5.4.
+
     The following values are obsolete:
 
     \value AA_ImmediateWidgetCreation This attribute is no longer fully