This removes the cube map setup and the draw scale parameter from the
tests that don't need them, and reuses the code for setting up the window
and the shader program for most of the texture tests. The tests are now
structured as follows:

TexCoordDrawTest: Test class that sets up a shader program for drawing
with texture coordinates. Vertex shader source can be overridden in
subclasses, and fragment shader source must be specified in subclasses.

Texture2DTest: Inherits TexCoordDrawTest, sets up a 2D texture and a
shader for drawing from it.

Texture2DTestWithDrawScale: Inherits Texture2DTest, adding a scale
parameter that scales the quad that gets drawn.

TextureCubeTest: Inherits TexCoordDrawTest, sets up a cube map and a 2D
texture and a shader for drawing from them.

Texture2DArrayTestES3: Inherits TexCoordDrawTest. Reserves a texture ID
and sets up an ESSL3 shader for drawing from a 2D texture array.

Also add a few comments about where things being tested are specified.

Also, ANGLETest::drawQuad parameter names are renamed to make their
meaning clearer. The parameters affect the vertex shader attribute
values, which the shader may use for other things besides setting the
vertex position.

BUG=angleproject:1261
TEST=angle_end2end_tests

Change-Id: Id673e36d5883aaaf47f2f830c2a1ad0ca293d578
Reviewed-on: https://chromium-review.googlesource.com/321620Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>