introduce GLTexture::clear and use it from paintredirector

also work around broken fbo texture clearing on fglrx

so far supports FBO/glClear and resorts to glTexSubImage2D

if the fbo cannot be created or is (in case of fglrx)
known to break, resort to glTexImage2D loading of an
argb array of zeros


FIXED-IN: 4.11.2
REVIEW: 112526