This refactoring makes the code for texture image upload significantly
smaller. It also aligns better with the layout of table 3.2 in the
OpenGL ES 3.0 specification which lists valid format/type combinations.

Custom RGB565 format handling was removed because it corresponds with a
regular 2-byte per pixel memcpy.

Sized internal formats were removed because a recent change fixed not
using the internalformat parameter as upload format.

Change-Id: Ief6c6e60c3d9a734cac241022bc95ffc88d752c8
Reviewed-on: https://swiftshader-review.googlesource.com/16568Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>