Increasing timeout for test.

With additional OpenGL checks, vtkRenderingOpenGLCxx-TestTextureSize  now takes
longer than 90s on certain machines (dash3). Increasing the timeout to 300s
which is typical for dashboards.

Change-Id: Iac4f5ff3f9eb22d3af1caf7507a162c7fa6cce05

Rendering/OpenGL/Testing/Cxx/CMakeLists.txt

@@ -178,4 +178,4 @@ endif()
 set_tests_properties(${vtk-module}Cxx-TestOpacity
   PROPERTIES TIMEOUT 70)
 set_tests_properties(${vtk-module}Cxx-TestTextureSize
-  PROPERTIES TIMEOUT 90)
+  PROPERTIES TIMEOUT 300)