Investigate pre-compiled shader programs and a shader disk cache with OpenGL ES 3.0+

Investigate pre-compiled shader programs (as in glProgramBinary in GLES 3.0 and up) in Qt.

Possible approaches:

• Use glGetProgramBinary to read back driver-specific linked programs. Could have a runtime disk cache in Qt (Quick), as already demonstrated on scenegraph-playground. This way the sources must still be shipped, and compilation happens on the first startup. ( + when driver / version string does not match anymore) Subsequent startups then just do a glProgramBinary and get the benefits.

• Some vendors provide host tools, like NVIDIA's glslc (proprietary, for embedded), which compile, link, and output glProgramBinary-suitable binaries.

• Other approaches don't really work (we cannot roll our own tool etc. due to (a) cross-compilation (b) the shader binaries being driver - and even driver version - specific.)

Cons:

• Batching uses shader rewriting therefore offline pre-compilation of shaders used by Qt Quick materials is not feasible.

• The OpenGL ShaderEffect's reflection is based on parsing shader sources. Offline pre-compiled shaders would require switching to GL program queries which are quite awful and pose various problems, like requiring a context.

Based on the above a runtime disk cache is the feasible solution, however:

• NVIDIA does shader caching on disk anyway, including on embedded.

• AMD too. WIP for Mesa? See https://www.phoronix.com/scan.php?page=news_item&px=Intel-Mesa-Shader-Cache-New

Might be wasted effort. Should benchmark first.