Fix clearing all samples of multisample render targets.

Only libGLESv2 was clearing all the samples of a multisample buffer.
Since all known APIs always clear all the samples, this could be
handled in the Renderer.

Bug swiftshader:77

Change-Id: Ib9adc3c61d263420ed0a0ae4828a693bd360b076
Reviewed-on: https://swiftshader-review.googlesource.com/10788
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>

src/Renderer/Surface.cpp

diff --git a/src/Renderer/Surface.cpp b/src/Renderer/Surface.cpp
index 1a63ff9..6bcc657 100644
--- a/src/Renderer/Surface.cpp
+++ b/src/Renderer/Surface.cpp
@@ -3193,14 +3193,14 @@
 		resource->unlock();
 	}
 
-	bool Surface::isEntire(const SliceRect& rect) const
+	bool Surface::isEntire(const Rect& rect) const
 	{
 		return (rect.x0 == 0 && rect.y0 == 0 && rect.x1 == internal.width && rect.y1 == internal.height && internal.depth == 1);
 	}
 
-	SliceRect Surface::getRect() const
+	Rect Surface::getRect() const
 	{
-		return SliceRect(0, 0, internal.width, internal.height, 0);
+		return Rect(0, 0, internal.width, internal.height);
 	}
 
 	void Surface::clearDepth(float depth, int x0, int y0, int width, int height)