Add Guillaume's screenshot patches for Mac. (Part of Stellarium#102)

src/StelMainView.cpp

@@ -1428,8 +1428,8 @@ void StelMainView::doScreenshot(void)
 	// First, image size:
 	glWidget->makeCurrent();
 	float pixelRatio = QOpenGLContext::currentContext()->screen()->devicePixelRatio();
-	int imgWidth =stelScene->width()  * pixelRatio;
-	int imgHeight=stelScene->height() * pixelRatio;
+	int imgWidth =stelScene->width();
+	int imgHeight=stelScene->height();
 	if (flagUseCustomScreenshotSize)
 	{
 		// Borrowed from Scenery3d renderer: determine maximum framebuffer size as minimum of texture, viewport and renderbuffer size
@@ -1477,7 +1477,7 @@ void StelMainView::doScreenshot(void)
 	QOpenGLFramebufferObjectFormat fbFormat;
 	fbFormat.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
 	fbFormat.setInternalTextureFormat(isGLES ? GL_RGBA : GL_RGB); // try to avoid transparent background!
-	QOpenGLFramebufferObject * fbObj = new QOpenGLFramebufferObject(imgWidth, imgHeight, fbFormat);
+	QOpenGLFramebufferObject * fbObj = new QOpenGLFramebufferObject(imgWidth * pixelRatio, imgHeight * pixelRatio, fbFormat);
 	fbObj->bind();
 	// Now the painter has to be convinced to paint to the potentially larger image frame.
 	QOpenGLPaintDevice fbObjPaintDev(imgWidth, imgHeight);
@@ -1491,7 +1491,7 @@ void StelMainView::doScreenshot(void)
 	sParams.viewportXywh[3]=imgHeight;
 
 	// Configure a helper value to allow some modules to tweak their output sizes. Currently used by StarMgr, maybe solve font issues?
-	customScreenshotMagnification=imgHeight/QApplication::desktop()->screenGeometry().height();
+	customScreenshotMagnification=(float)imgHeight/QApplication::desktop()->screenGeometry().height();
 
 	sParams.viewportCenter.set(0.0+(0.5+pParams.viewportCenterOffset.v[0])*imgWidth, 0.0+(0.5+pParams.viewportCenterOffset.v[1])*imgHeight);
 	sParams.viewportFovDiameter = qMin(imgWidth,imgHeight);