spicec: fix non-doublebuffer drawing

First, context must set it, then Draw/ReadBuffer must be set to FRONT,
and then explicit Flush is needed.

This patch is mostly for future reference, it is mostly discarded in
following patch using double-buffer.

client/x11/platform.cpp

@@ -3224,6 +3224,7 @@ void Platform::init()
         int num_configs;
         int attrlist[] = {
             GLX_RENDER_TYPE, GLX_RGBA_BIT,
+            GLX_DOUBLEBUFFER, False,
             GLX_DRAWABLE_TYPE, GLX_PBUFFER_BIT | GLX_WINDOW_BIT,
             GLX_X_VISUAL_TYPE, GLX_TRUE_COLOR,
             GLX_RED_SIZE, 8,

client/x11/red_drawable.cpp

@@ -85,6 +85,7 @@ static inline void copy_to_gldrawable_from_gltexture(const RedDrawable_p* dest,
     if (rendertype == RENDER_TYPE_FBO) {
         GLuint fbo;
 
+        glFlush();
         fbo = source->gl.fbo;
         glBindFramebuffer(GL_FRAMEBUFFER_EXT, fbo);
     } else {

client/x11/red_pixmap_gl.cpp

@@ -79,6 +79,8 @@ RedPixmapGL::RedPixmapGL(int width, int height, RedDrawable::Format format,
             glXDestroyContext(XPlatform::get_display(), _glcont);
             THROW("no GL_EXT_framebuffer_object extension");
         }
+        glDrawBuffer(GL_FRONT);
+        glReadBuffer(GL_FRONT);
 
         glGenTextures(1, &tex);
         glBindTexture(GL_TEXTURE_2D, tex);