On EGL always clear all buffers

In egl chips like SGX and MBX clearing all buffers is a signal for the driver that we're starting a new frame and that the previous one can be thrown away. The buffers should be cleared even if they were not used. Without this we're suffering from a very noticable perf penalty.

This patch makes the buffers always be cleared on ::beginPaint - when compiling with EGL. It should prolly be a compile-time setting/define but I think it actually makes sense to be a default for EGL.