Add support for linear gradients.

Change-Id: Id15329da065045b3f06fdaed615f33cd57608496
diff --git a/libs/hwui/Program.cpp b/libs/hwui/Program.cpp
index 6202ba3..841b6c8 100644
--- a/libs/hwui/Program.cpp
+++ b/libs/hwui/Program.cpp
@@ -190,6 +190,8 @@
     gradient = addUniform("gradient");
     gradientLength = addUniform("gradientLength");
     sampler = addUniform("sampler");
+    start = addUniform("start");
+    screenSpace = addUniform("screenSpace");
 }
 
 void DrawLinearGradientProgram::use() {