Sync with master

Adding GPU_GetVirtualResolution

This fixes Issue #11

* official:
Updated renderer-test for the previous change.
Changed renderer's shader_version to min_shader_version and max_shader_version for more flexibility when considering GPU capability.
Removed SDL_PREALLOC hack and stbi allocator override. Replaced them with a copy of the data instead to try to avoid crashes with Visual Studio. Lots of whitespace changes because I forgot to configure my editor first.

Raspberry Pi is a armv7-a cortex-a7 with NEON. It's default OS is
Raspbian which is a Debian offshoot. From a build environment
perspective, it is almost indistiguishable from any other Debian based
Linux.

Pi supports OpenGL ES 2.0. Some of the code assumed iOS or Android, so
this needed to be fixed.

It appeared link_libraries was giving me problems early on, so I changed
to target_link_libraries. I'm not sure if this change is still needed,
but link_libraries is deprecated, so it's probably a good change
regardless.

There seems to be a bug in the Pi libraries for glBlendEquationOES,
glBlendEquationSeparateOES, glBlendFuncSpearateOES. Despite the
headers, these symbols are missing in the library, but exist
without the OES suffix. A hack to use the non-OES names is
employed.

Changed CMAKE_SOURCE_DIR to CMAKE_CURRENT_SOURCE_DIR to allow CMake script to be included from others.