File renaming in preparation for dropping from OpenGL 3.3 to OpenGL 2.1.
Drop the shader version in the default renderer down to the earliest version.

This is needed in order for OpenGL 2.1 to work on 2.1 class hardware. Technically, we should set the version to 120, but we're not doing anything where it really matters.
Change the requested OpenGL version number from 3.3 to 2.1.
Have the OpenGL 2.1 renderer output warnings in addition to errors.

This is untested on AMD.
Fix some shaders for the new rendering system.
A big commit for getting the engine just barely running on Intel HD 3000 GPUs.

There are still a few issues remaining here:
 - Some shaders are still using GLSL 1.3+ features
 - ARB_texture_rg support is assumed
 - ARB_texture_float support is assumed

This is work towards addressing issue #60.
Fix a framebuffer issue with the OpenGL 2.1 renderer.
Add prototypes for Gaussian blur shader generators.

This new system is needed in order to make Gaussian blurring a little less cumbersome and generalized. Also, it'll make it easier to allow customizations. For example, the blur kernels can potentially be reduced or increased for individual shadow-casting lights.

Also, this is going to be GLSL 110 compliant which is going to be a partial fix for the OpenGL 2.1 renderer.
Add untested implementation of Gaussian blur shader generators.
Fix the new blur shaders and start using explicitly using GLSL 1.2.

Using "#version 120" is nice because it avoids some annoying warnings on Intel drivers. Also, since we're targeting OpenGL 2.1, it's not really a problem.
Cleanup and restructure.
Fix an issue where a GLSL function function is being used that's not available in version 120.
Remove the hard-coded blur shaders.
Move the depth pass shader to the shader builder.
Restructure some shader code to encourage the use of a MAD instruction.
Move the highlight shader of the default scene renderer over the shader builder.
Move shadow map shaders for the default renderer over to the shader builder.
Move the remaining shaders for the default renderer over to the shader builder.

With this commit, there is no longer any shader code in the "shaders" directory for the default renderer. Future plans are to get rid of all external engine shader code to simplify the engine's distribution package.
Renaming.