when compiled with GStreamer, open (of a file) segfaults.

Fix was suggested by Bostjan Vesnicer.

Also fixed a typo in performance testing framework and removed hardcoded
temporary file name from highgui perf test

The change is based on pull request #211.

This reverts commit 2e7656db.

* removed OpenGLFuncTab, now extensions are loaded internally
* renamed GlTexture -> GlTexture2D
* added support of GlBuffer and GlTexture2D to InputArray/OutputArray
* added ELEMENT_ARRAY_BUFFER and PIXEL_PACK_BUFFER targets
* added copyFrom/copyTo method for GlBuffer and GlTexture2D
* removed GlFont
* removed pointCloudShow
* removed OpenGLCleanCallback

- Added Gain control
- Changed default acquisition mode to grayscale
- Fixed a signed/unsigned compare issue
- Implemented frame resizing
- Implemented automatic buffer resizing on frame resize
- Corrected camera initialization so that it attempts to put the camera in Mono8
  but does not fail if this doesn't succeed.

These codes should be included into regular builds.

disabled several tests on Mac when no FFMPEG is used; disabled automatic table initialization in imgwarp in the case of MinGW

+ Some minor formatting fixes.

This reverts commit 36135fc5.

Window-related functions is guarded from calls from application code.
Old implementations with "no-gui" exception are stell compiled and linked to OpenCV library for binary compatibility.