Add a DXGI format for GL_LUMINANCE16F_EXT in ES3.

BUG=angle:642

Change-Id: I41eb64b8e9963cfe33e4c127f84a16eef9b118d3
Reviewed-on: https://chromium-review.googlesource.com/198379
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libGLESv2/renderer/loadimage.cpp b/src/libGLESv2/renderer/loadimage.cpp
index a36ce58..6e2d246 100644
--- a/src/libGLESv2/renderer/loadimage.cpp
+++ b/src/libGLESv2/renderer/loadimage.cpp
@@ -209,7 +209,7 @@
                 dest[4 * x + 0] = source[x];
                 dest[4 * x + 1] = source[x];
                 dest[4 * x + 2] = source[x];
-                dest[4 * x + 3] = 0x3C00; // SEEEEEMMMMMMMMMM, S = 0, E = 15, M = 0: 16bit flpt representation of 1
+                dest[4 * x + 3] = gl::Float16One;
             }
         }
     }