On desktop GL, using an internal format of GL_RGBA produces GL_RGBA8
textures even if format is set to GL_FLOAT.  Use sized internal formats
whenever possible to ensure that the internal format of the texture is
the expected format.

Fixes:
 * conformance/extensions/oes-texture-float-with-image-data.htm
 * conformance/extensions/oes-texture-float-with-image.html
 * conformance/extensions/oes-texture-float.html
 * conformance/extensions/oes-texture-half-float-linear.html
 * conformance/extensions/oes-texture-half-float-with-image-data.html
 * conformance/extensions/oes-texture-half-float-with-image.html
 * conformance/extensions/oes-texture-half-float.html

BUG=angleproject:884

Change-Id: I48f220a4da518fff5831155a45c04cafcdffd33b
Reviewed-on: https://chromium-review.googlesource.com/288350Tested-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>