Fixed support in the vulkan backend for legacy
GLES2.0 formats (luminance, alpha,
luminance_alpha)

Correctly exposed the following extensions:
OES_texture_float
OES_texture_half_float
OES_texture_float_linear
OES_texture_half_float_linear
EXT_color_buffer_float
EXT_color_buffer_half_float

Some of the above extensions have different
requirements depending on other extension support
and the context client version, and were
incorrectly assuming the most restrictive
requirements to be exposed.

Implemented end2end tests for:
OES_texture_float
OES_texture_half_float
OES_texture_float_linear
OES_texture_half_float_linear
EXT_color_buffer_float
EXT_color_buffer_half_float

Bug: angleproject:2898
Bug: angleproject:2726
Test: ./angle_end2end_tests --gtest_filter='Texture2DFloatTest*'
Change-Id: I7024aa1393eadafb5a0fb83c23e9035aae650b67
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1740276
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>