Don't define highp/mediump/lowp if desktop GL has them

OpenGL 4.0 systems now have compatibility with ES2, including
support for the precision qualifiers.  If the GL_ARB_ES2_compatibility
extension is present, then we don't define highp/mediump/lowp
to the empty string.

Task-number: QTBUG-12862
Reviewed-by: Sarah Smith

Author: Rhys Weatherley

dist/changes-4.6.4

@@ -63,8 +63,9 @@ QtNetwork
 QtOpenGL
 --------
 
- - foo
-    * bar
+ - QGLShaderProgram
+    * [QTBUG-12862] Don't #define highp/mediump/lowp if the desktop OpenGL
+      implementation has the GL_ARB_ES2_compatibility extension.
 
 QtScript
 --------

src/opengl/qgl.cpp

@@ -4923,6 +4923,8 @@ QGLExtensions::Extensions QGLExtensions::currentContextExtensions()
         glExtensions |= FragmentProgram;
     if (extensions.match("GL_ARB_fragment_shader"))
         glExtensions |= FragmentShader;
+    if (extensions.match("GL_ARB_ES2_compatibility"))
+        glExtensions |= ES2Compatibility;
     if (extensions.match("GL_ARB_texture_mirrored_repeat"))
         glExtensions |= MirroredRepeat;
     if (extensions.match("GL_EXT_framebuffer_object"))
@@ -4941,6 +4943,7 @@ QGLExtensions::Extensions QGLExtensions::currentContextExtensions()
     glExtensions |= FramebufferObject;
     glExtensions |= GenerateMipmap;
     glExtensions |= FragmentShader;
+    glExtensions |= ES2Compatibility;
 #endif
 #if defined(QT_OPENGL_ES_1) || defined(QT_OPENGL_ES_1_CL)
     if (extensions.match("GL_OES_framebuffer_object"))

src/opengl/qgl_p.h

@@ -291,7 +291,8 @@ class QGLExtensions {
         DDSTextureCompression   = 0x00008000,
         ETC1TextureCompression  = 0x00010000,
         PVRTCTextureCompression = 0x00020000,
-        FragmentShader          = 0x00040000
+        FragmentShader          = 0x00040000,
+        ES2Compatibility        = 0x00080000
     };
     Q_DECLARE_FLAGS(Extensions, Extension)
 

src/opengl/qglshaderprogram.cpp

@@ -97,6 +97,10 @@ QT_BEGIN_NAMESPACE
     to just features that are present in GLSL/ES, and avoid
     standard variable names that only work on the desktop.
 
+    If the \c{GL_ARB_ES2_compatibility} extension is present,
+    then the above prefix is not added because the desktop OpenGL
+    implementation supports precision qualifiers.
+
     \section1 Simple shader example
 
     \snippet doc/src/snippets/code/src_opengl_qglshaderprogram.cpp 1
@@ -390,8 +394,10 @@ bool QGLShader::compileSourceCode(const char *source)
             srclen.append(GLint(headerLen));
         }
 #ifdef QGL_DEFINE_QUALIFIERS
-        src.append(qualifierDefines);
-        srclen.append(GLint(sizeof(qualifierDefines) - 1));
+        if (!(QGLExtensions::glExtensions() & QGLExtensions::ES2Compatibility)) {
+            src.append(qualifierDefines);
+            srclen.append(GLint(sizeof(qualifierDefines) - 1));
+        }
 #endif
 #ifdef QGL_REDEFINE_HIGHP
         if (d->shaderType == Fragment) {