hwui: add extension to support unpack operations in OpenGL ES 2.0

OpenGL ES 3.0+ lets us specify the row length for unpack operations
such as glTexSubImage2D(). This allows us to upload a sub-rectangle
of a texture. Also, the GL_EXT_unpack_subimage extension can also
support this feature in OpenGL ES 2.0

Change-Id: Id43c2c55c5eaefbace67087c955f0b4324fb2c35
Signed-off-by: xiaozhengdong <xiaozhengdong@xiaomi.com>