Fix detection of pbuffers on OpenGL/ES systems
Fix detection of pbuffers on OpenGL/ES systems

The previous code was searching for an exact pbuffer format of
RGBA = 1, 1, 1, 0, which of course is never going to happen.
Instead, search for the best format.

Reviewed-by: trustme

Conflicts:

	src/opengl/qglpixelbuffer_egl.cpp

Back-port of 46843022acd7322c42a98858ec52b65ce7451d06

radekp/qt,pruiz/wkhtmltopdf-qt,radekp/qt,RLovelett/qt,radekp/qt,pruiz/wkhtmltopdf-qt,RLovelett/qt,RLovelett/qt,KDE/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,igor-sfdc/qt-wk,KDE/qt,KDE/qt,pruiz/wkhtmltopdf-qt,radekp/qt,radekp/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,radekp/qt,RLovelett/qt,radekp/qt,RLovelett/qt,KDE/qt,RLovelett/qt,radekp/qt,KDE/qt,RLovelett/qt,RLovelett/qt,RLovelett/qt,radekp/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,KDE/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,radekp/qt,KDE/qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,KDE/qt,pruiz/wkhtmltopdf-qt,RLovelett/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,RLovelett/qt,pruiz/wkhtmltopdf-qt