libagl: add support for GL_BGRA

glReadPixel data need BGRA type for Atmel SAM overlay buffer data.
add support for type BGRA.

Signed-off-by: Royce Jiang <jiangxd@embedinfo.com>
screencap: support screencap the whole screen

Add support for surfaceflinger to capture the whole screen without
Framebuffer Objects GL Extension.

Signed-off-by: Royce Jiang <jiangxd@embedinfo.com>