The GL_EXT_color_buffer_float spec
https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_color_buffer_float.txt
mentions that some of the formats must be blendable:
"Blending applies only if the color buffer has a fixed-point or
 floating-point format. If the color buffer has an integer format,
 proceed to the next operation. Furthermore, an INVALID_OPERATION
 error is generated by DrawArrays and the other drawing commands
 defined in section 2.8.3 (10.5 in ES 3.1) if blending is enabled
 and any draw buffer has 32-bit floating-point format components."

So any non 32-bit floating-point must be blendable.

Bug: angleproject:4291
Change-Id: I850fb13b71e4ad03fd07e2a265896c9106bc0ce1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2001478
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>