KWin already has a de facto OpenGL 2 dependency through QML. Combined
with the fact that the OpenGL 1 backend is basically unmaintained and
also unused, it's better to remove it for the new major release.

This change includes:
 * Removal of cmake option KWIN_BUILD_OPENGL_1_COMPOSITING
 * Removal of KWIN_HAVE_OPENGL_1 compile option and all code
   ifdef'ed with it (partially removal of if-else constructs)
 * Removal of CompositingType::OpenGL1Compositing (flags are kept
   as a core flag should get introduced)
 * Driver recommendation for OpenGL1Compositing changed to XRender
   (should be evaluated whether the drivers can provide GL2)
 * Removal of configuration option "GLLegacy"
 * Removal of fooMatrix function in kwinglutils
 * Removal of ARBBlurShader
 * Removal of legacy code path in GLVertexBuffer
 * Removal of GLShaderManager::disable
 * if-blocks with ShaderManager::instance()->isValid() removed

REVIEW: 116042