dEQP uses the following rules for unsized float format renderability in
ES2:

HALF_FLOAT_OES + RED : GL_EXT_texture_rg and GL_OES_texture_half_float
HALF_FLOAT_OES + RG  : GL_EXT_texture_rg and GL_OES_texture_half_float
HALF_FLOAT_OES + RGB : GL_EXT_color_buffer_half_float and GL_OES_texture_half_float
HALF_FLOAT_OES + RGBA: GL_EXT_color_buffer_half_float and GL_OES_texture_half_float

FLOAT + RED : GL_EXT_texture_rg and GL_OES_texture_float
FLOAT + RG  : GL_EXT_texture_rg and GL_OES_texture_float
FLOAT + RGB : never
FLOAT + RGBA: never

BUG=angleproject:3283

Change-Id: I4c4c0d290d2c7dec45419a0765fa27b79f0ad774
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1773304Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>