[Piglit] More tests for format conversions in ARB_texture_view

[Chris Forbes]

Here are two more tests which ensure that all formats in each view class convert correctly. They are testing similar things, but via different mechanisms and have found different bugs.

1) Create views with all formats in each view class, and ensure that for each format,
texel data read back via glGetTexImage (with the pixel format and type specified in ARB_shader_image_load_store for handling format mismatches) is identical.

2) For each format, compare sampling from a view to sampling from a texture created with that format.

Compressed formats are not covered -- there's no interesting component-ordering variations.