[Feat] (MG_State, MG_Impl): give program pipelines their object and their state

Every program pipeline entry point was an export stub, and the stub macro's
`return (type)1` made glIsProgramPipeline answer GL_TRUE for anything - including
the names glGenProgramPipelines had never written. All four
direct_state_access.program_pipelines cases failed.

ProgramPipelineObject holds what GL 4.6 core 7.4 says a pipeline is: a program
reference per shader stage, the active program glProgramUniform* addresses, a
validate status and an info log. Its validate status starts false, unlike
ProgramObject's, because a pipeline that has never been validated must report
GL_VALIDATE_STATUS as 0.

The name rules follow the shape queries and transform feedbacks already use, and
which the CTS checks first: glGenProgramPipelines only RESERVES a name and
glIsProgramPipeline answers GL_FALSE for it; the object appears on first bind, or
immediately from glCreateProgramPipelines. Map membership is object existence -
a pipeline, unlike a transform feedback, has no stateful default object zero, so
no everBound flag is needed.

glGet(GL_PROGRAM_PIPELINE_BINDING) reports the real binding now instead of a
hardcoded zero whose comment said the entry points were stubbed.

This is the state half only. program_pipelines_functional needs mixed-stage
rendering - a vertex-only and a fragment-only program drawn together - and stays
failing; glCreateShaderProgramv is deliberately left stubbed until that lands, so
nothing can half-work in between.

Takes program_pipelines_creation, _defaults and _errors from failing to passing
on both backends.

CMakeLists.txt

@@ -218,6 +218,7 @@ set(SOURCE_FILES
     MobileGL/MG_Impl/GLImpl/Framebuffer/Validators.cpp
     MobileGL/MG_Impl/GLImpl/Framebuffer/GL_Framebuffer.cpp
     MobileGL/MG_Impl/GLImpl/Program/GL_Program.cpp
+    MobileGL/MG_Impl/GLImpl/Program/GL_ProgramPipeline.cpp
     MobileGL/MG_Impl/GLImpl/Texture/GL_Texture.cpp
     MobileGL/MG_Impl/GLImpl/Texture/Validators.cpp
     MobileGL/MG_Impl/GLImpl/Texture/ProxyTexture.cpp