SPIRV: Add image instruction support and atomic counter support.

the expectations of old GLSL shadow*() lookups.

Structured control-flow rules allow leaving the middle of a construct through
a return, but not through a jump to a block that does a return.

Addresses issue #58.

This generally simplifies access chain generation, with far fewer  type conversions.
It is particularly important to future SPIR-V changes where there is less aggregate
type uniqueness due to carrying different layout information with the type.

This includes run-time block.member.length() (OpArrayLength).

Front-end Arrays of arrays: Add ES-specific checks and tests.  AoA should be quite close to done now.

Sync local master branch from the upstream

SPIR-V: Add support for texelFetch

SPIRV: Decorate matrices and arrays with their strides

cmake: Don't override CMAKE_INSTALL_PREFIX

Preprocessor:  'defined' cannot be used as a macro name in #define or #undef.

There's nothing really special about texelFetch other than the opcode and
some restrictions so adding it is pretty trivial.

Stop forcing CMAKE_INSTALL_PREFIX="install". If the user manually set
CMAKE_INSTALL_PREFIX, then trust that he knows what he's doing.

This patch does NOT change the project's default value ("install") of
CMAKE_INSTALL_PREFIX.

Change-Id: I81b46dd1986427b498fe6316bed03f01689987d4

"No layout(location=) is required if there is only one output" should not count
built-in variables like gl_SampleMask.  This is fixed.

Backward incompatible: Turn on PureOperatorBuiltins: use only enum-based built-in functions in the AST.

If this breaks your AST consumer, best is to modify it to test
against the enum values instead of doing string comparisons on
built-in function names.  This is the reason the change was made.

If you need the old behavior, you should be able to get it back by changing
PureOperatorBuiltins to be false instead of true.  This path will work for
a while, but is marked deprecated.

Also, the old behavior is tagged as release 2.4.