Added a changelog.

Author: svenpanne

CHANGELOG.md

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+2.12.1.0
+--------
+* Added missing drawing commands using vertex arrays and some related types.
+* Added missing whole framebuffer operations.
+* Added getters for `stencilMaskSeparate`, `stencilFuncSeparate`, and `stencilOpSeparate`, making them full-blown `StateVar`s.
+* Added `patchDefaultInnerLevel` and `patchDefaultOuterLevel` to control the default tessellation levels.
+* Added `ContextLost` constructor to `ErrorCategory` type.
+* Added `SeparateAttribs` and deprecated `SeperateAttribs`, fixing a spelling mistake.