Mesa: improvements for use with Qt5

Among other changes, this patch makes it possible to let Mesa render into
a user-provided buffer instead of the screen. This can be achieved with
the 'eglCreateWindowSurface()' function, which takes a buffer
description as third argument.

Fixes #974.
base-linux: round up RM session size to page size

Since RM sessions can be used as dataspaces and dataspace sizes are
supposed to have page granularity, RM session sizes should have page
granularity, too.

Fixes #972.
libc: fix size calculation in 'Mem_alloc_impl::alloc'

When allocating new backing store in 'Mem_alloc_impl::alloc()', the
requested alignment needs to be considered in the size calculation.

Fixes #973.
Qt5: enable OpenGL support

Known limitations:

- only one Qt window can use OpenGL at this time
- window resizing using the resize handle does not work

Fixes #975.