Clean up ensureRenderTarget().

We can use the same code for this method, with the only difference
being in when we determine a texture image holds no pixels.

BUG=angle:729

Change-Id: I3356f5deb900c24b1a5c739ce8de99e9ce1371c0
Reviewed-on: https://chromium-review.googlesource.com/219861
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libGLESv2/renderer/Image.cpp b/src/libGLESv2/renderer/Image.cpp
index 370b086..40721f3 100644
--- a/src/libGLESv2/renderer/Image.cpp
+++ b/src/libGLESv2/renderer/Image.cpp
@@ -4,7 +4,7 @@
 // found in the LICENSE file.
 //
 
-// Image.h: Implements the rx::Image class, an abstract base class for the 
+// Image.h: Implements the rx::Image class, an abstract base class for the
 // renderer-specific classes which will define the interface to the underlying
 // surfaces or resources.
 
@@ -15,7 +15,7 @@
 
 Image::Image()
 {
-    mWidth = 0; 
+    mWidth = 0;
     mHeight = 0;
     mDepth = 0;
     mInternalFormat = GL_NONE;