Fix image export on *nix.

Before this commit, trying to export image on *nix platforms yielded
a black rectangle, since since there is nowhere to render to
when we're not in a GUI toolkit draw callback.

On Windows, nothing changes: we do a repaint without the toolbar,
glReadPixels, export. On *nix, we create another offscreen rendering
context, render into it, then destroy it. As a bonus this avoids
some minor flickering that would happen if we reused the regular
rendering path.

src/export.cpp

@@ -7,6 +7,9 @@
 // Copyright 2008-2013 Jonathan Westhues.
 //-----------------------------------------------------------------------------
 #include "solvespace.h"
+#ifndef WIN32
+#include <unix/gloffscreen.h>
+#endif
 #include <png.h>
 
 void SolveSpaceUI::ExportSectionTo(const std::string &filename) {
@@ -1048,6 +1051,10 @@ void SolveSpaceUI::ExportAsPngTo(const std::string &filename) {
     // so repaint the scene. And hide the toolbar too.
     bool prevShowToolbar = SS.showToolbar;
     SS.showToolbar = false;
+#ifndef WIN32
+    std::unique_ptr<GLOffscreen> gloffscreen(new GLOffscreen);
+    gloffscreen->begin(w, h);
+#endif
     SS.GW.Paint();
     SS.showToolbar = prevShowToolbar;
 

src/solvespace.h

@@ -17,6 +17,7 @@
 #include <math.h>
 #include <limits.h>
 #include <algorithm>
+#include <memory>
 #include <string>
 #include <vector>
 #include <map>

src/unix/gloffscreen.cpp

@@ -27,6 +27,8 @@ GLOffscreen::GLOffscreen() : _pixels(NULL), _pixels_inv(NULL), _width(0), _heigh
 }
 
 GLOffscreen::~GLOffscreen() {
+    delete[] _pixels;
+    delete[] _pixels_inv;
     glDeleteRenderbuffersEXT(1, &_depth_renderbuffer);
     glDeleteRenderbuffersEXT(1, &_color_renderbuffer);
     glDeleteFramebuffersEXT(1, &_framebuffer);