Added new elliptical gradient fill method and updated vbackground command.
Renamed enum values of Aspect_GradientFillMethod and defined aliases for old ones.
Changed draw mode in OpenGl_BackgroundArray from triangle-strip to GL_TRIANGLES.
Removed testgrids 'bugs/vis' and '3rdparty'; tests have been redistributed across other grids.
Moved testgrid 'v3d/ivtk' into dedicated 'vtk/ivtk'.
Added testgrid 'vselect' dedicated to 3D viewer picking/selection functionality
and filled with tests from 'v3d/vertex', 'v3d/face' and similar groups.
Added testgrid 'opengl' dedicated to OpenGL driver low-level functionality (GLSL programs and similar)
and filled with tests from 'v3d/glsl', 'v3d/raytrace', '3rdparty/fonts', 'bugs/vis' (portion) and similar.
Added testgrid 'opengles3' dedicated to OpenGL ES 3.0 driver low-level functionality
and reusing tests from 'opengl' testgrid.
Subgroup 'opengles3/raytrace' is disabled on Windows, as Ray-Tracing currently requires OpenGL ES 3.2.
while ANGLE library implements only OpenGL ES 3.0.
Added testgrid 'opengles2' dedicated to OpenGL ES 2.0 driver low-level functionality
and reusing a limited subset of passing tests from 'opengl' testgrid.
Currently testgrid is activated only on Windows platform when using ANGLE library
(properietary OpenGL ES drivers do not allow creation of restricted 2.0 context).
Test cases have been cleaned out to put bug description into log,
to properly load necessary plugins and to explicitly dump viewer.
