* Add GPU-accelerated heatmaps
* Add BustPlotCache() and other small changes
* Fixupside-down heatmaps
* Prevent new/delete on each call to PlotHeatmap() on certain cases
* Add common interface for backends & move context to ImPlotContext
* Fix integer heatmaps not working properly
* Misc changes
- Rename implot_gpu.h -> implot_backend.h
- Rename IMPLOT_ENABLE_OPENGL3_ACCELERATION -> IMPLOT_BACKEND_ENABLE_OPENGL3
- Use gp.CurrentItem->ID over gp.CurrentPlot->ID
- Add flags for different features supported by backends
* Cleanup & maintenance
- Removed unnecessary attributes from shader
- Renamed functions & structs
- Reduced memory footprint of OpenGL backend context
* Standandardize backends for future implementations
* Added logarithmic X & Y axes
* Fix vertically-flipped 2D histograms
* Fix incorrect colormap colors
