Support compilation of multiple DVLEs into a single DVLB

[neobrain]

The DVLB format supports the definition of multiple shaders (DVLEs). Currently, nihstro supports only generation of DVLBs with a single DVLE, which is created solely from the input source file. Anything more advanced is a nontrivial task, because it's not entirely clear how to approach this.

Workflows which should be supported:

• Generating a DVLB containing a single DVLE obtained from a single source file
• Generating a DVLB containing multiple DVLEs obtained from a single source file using different compile settings (e.g. entry point, macros, definitions, ...)
• Generating a DVLB containing multiple DVLEs, each of which was obtained from a separate source file
• Any combination of the things above

Additional features to consider:

• common-code detection. This will become particularly important whenever common assembly code is included through .include.
• dead-code elimination. This will become important for "uber-shaders" which implement lots of things, but only use a small subset for each entry-point.

I thought about this issue for a while, and I think the most sensible approach to solve it is the following:

• Introduce a separated compiler-linker model: Source code is compiled to a DVOJ, multiple DVOJs are then linked into a DVLB.
• Compile each input source file once per input configuration (i.e. compile a file multiple times if it's supposed to be compiled with different entry points).
• Finally, link all compiled sources together: Since each DVLB may only contain one code binary, this involves relocating each compiled shader blob to a particular offset. To make sure we don't waste too much space (the shader binary is limited to something in the order of 1024 words), dead-code elimination and common-code detection will be required. These may be omitted for an initial implementation, however.