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/GrRenderTarget.cpp b/src/gpu/GrRenderTarget.cpp
index 13fc229..505cc78 100644
--- a/src/gpu/GrRenderTarget.cpp
+++ b/src/gpu/GrRenderTarget.cpp
@@ -68,7 +68,7 @@
     if (kUnknown_GrPixelConfig == fDesc.fConfig) {
         colorBits = 32; // don't know, make a guess
     } else {
-        colorBits = GrBytesPerPixel(fDesc.fConfig);
+        colorBits = GrBytesPerPixel(fDesc.fConfig) * 8;
     }
     uint64_t size = fDesc.fWidth;
     size *= fDesc.fHeight;