32 bpp floating point textures

This is VERY preliminary, but it was sufficient for me to get 32 bit floating point textures in a sample app

BUG=skia:
R=bsalomon@chromium.org, bsalomon@google.com

Author: joshualitt@chromium.org

Review URL: https://codereview.chromium.org/359803003
diff --git a/src/gpu/gl/GrGLDefines.h b/src/gpu/gl/GrGLDefines.h
index 1e8d7cc..77b7a15 100644
--- a/src/gpu/gl/GrGLDefines.h
+++ b/src/gpu/gl/GrGLDefines.h
@@ -731,6 +731,7 @@
 #define GR_GL_RGB5_A1                        0x8057
 #define GR_GL_RGB565                         0x8D62
 #define GR_GL_RGBA8                          0x8058
+#define GR_GL_RGBA32F                        0x8814
 #define GR_GL_RGB8                           0x8051
 #define GR_GL_BGRA8                          0x93A1
 #define GR_GL_SRGB                           0x8C40