Slightly simplifies the upcoming refactor

Unlike the hardware backends, the software renderer can use multiple scissor rectangles (though this will result in extra rasterization).

This moves much of the duplicated bounding box code into VideoCommon,
leaving only the specific buffer implementations in each backend.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

MAX_XFB_WIDTH/HEIGHT are the largest XFB sizes seen in practice, but do not make sense to use for the backbuffer size, which should be the size of the window.  The old code created screenshots with a size of 720x540 on NTSC games when "Dump Frames at Internal Resolution" is unchecked; now, the window size is used.

This will allow for some aspects of bounding box to be handled in
VideoCommon instead of individual backends.

Renderer: Adjust source rectangle when crop would draw off screen 

Greatly simplifies the overall interface when it comes to compiling
shaders. Also allows getting rid of a std::string overload of the same
name. Now std::string and const char* both go through the same function.