better workaround for Intel GPU crash for glDrawBuffer on Linux

fix a linux build issue

btgui/OpenGLWindow/X11OpenGLWindow.cpp

@@ -15,6 +15,7 @@
 #include <unistd.h>
 
 #include <pthread.h>
+extern bool gIntelLinuxglDrawBufferWorkaround;
 
 GLint                   att[] = { GLX_RGBA, GLX_DEPTH_SIZE, 24, GLX_DOUBLEBUFFER, None };
 static bool forceOpenGL3 = true;
@@ -233,6 +234,12 @@ void X11OpenGLWindow::enableOpenGL()
     }
     const GLubyte* ven = glGetString(GL_VENDOR);
     printf("GL_VENDOR=%s\n", ven);
+
+    if (strncmp((const char*)ven,"Intel",5)==0)
+	{
+		printf("Workaround for some crash in the Intel OpenGL driver on Linux/Ubuntu\n");
+		gIntelLinuxglDrawBufferWorkaround=true;
+	}
     const GLubyte* ren = glGetString(GL_RENDERER);
     printf("GL_RENDERER=%s\n",ren);
     const GLubyte* ver = glGetString(GL_VERSION);