citra-qt: Add customizable speed limit target
* Update SDL config for the new frame_limit option
* Made max lag time a function of target speed percent.
* Added a checkbox to enable/disable frame limiter
* UI: Prevent frame_limit from under/overflowing
* UI: Hide target speed percent when frame limiter is off
* Disable frame limit spin box when framelimit isn't enabled
The OpenGL renderer has been revised, with the following changes:
- Initialization and rendering have been refactored to reduce the number of
redundant objects used.
- Framebuffer rotation is now done directly, using texture mapping.
- Vertex coordinates are now given in pixels, and the projection matrix
isn't hardcoded anymore.
