This PR adds support for specialization constants for the OpenGL
backend. The minimum OpenGL version we are targetting doesn't
have native support for specialization constants. We simulate this
by keeping track of shader programs for each set of specialization
constants that are being used.
Specialization constants can be used to reduce shader complexity
and improve performance as less registry and/or spilling is done.
This requires the ability to recompile GLShaders. In order to do this
we need to keep track of the sources that are used when the shader
was compiled. For static sources we only store references
(`GLSource::source_ref`), for dynamically generated sources we keep
a copy of the source (`GLSource::source`).
When recompiling the shader GLSL source-code is generated for the
constants stored in `Shader::constants`. When compiling the previous
GLSource that contains specialization constants is then replaced
by the new version.
Pull Request: https://projects.blender.org/blender/blender/pulls/116926
4ac0267567