Added metrics gathering for GL calls

Added metrics gathering for GL calls

[solution]     Added a gl proxy implementation class that overrides the methods
that we're interested in gathering metrics on, and only use this proxy implementation
if the DALI_GRAPHICS_STATUS_TIME environment variable is set.
Added calls from RenderThread to use the new PreRender and PostRender methods in the
GL abstraction -this enables the proxy implementation to count per-frame data.

Change-Id: I25e09664f6b2f3fd9a6b33f99622d547362e2849
Signed-off-by: David Steele <david.steele@partner.samsung.com>
Signed-off-by: Adeel Kazmi <adeel.kazmi@samsung.com>