Reset mUnmodifiedDataUse after bufferSubData

Without this, enough draws from a buffer modified only with bufferSubData will cause it to switch become static.

BUG=
TEST=

Review URL: http://codereview.appspot.com/5123042

git-svn-id: https://angleproject.googlecode.com/svn/trunk@769 736b8ea6-26fd-11df-bfd4-992fa37f6226
diff --git a/src/libGLESv2/Buffer.cpp b/src/libGLESv2/Buffer.cpp
index 2cd148e..dd12e3c 100644
--- a/src/libGLESv2/Buffer.cpp
+++ b/src/libGLESv2/Buffer.cpp
@@ -74,6 +74,8 @@
     {
         invalidateStaticData();
     }
+
+    mUnmodifiedDataUse = 0;
 }
 
 StaticVertexBuffer *Buffer::getStaticVertexBuffer()