fix(gl): gate fallible constructors on a complete loader table

- loader: add available(), true only after load() has resolved every
  required entry point (false at the start of load()). Program::from_source
  and the other fallible constructors return ErrorCode::Unsupported while it
  is false, so a failed load() with a partially populated table can no longer
  fault on an unresolved entry point.
- caps: GL_CONTEXT_PROFILE_MASK only exists from 3.2, so query it only on
  3.2+ contexts. A 3.0/3.1 query latches GL_INVALID_ENUM in the context, which
  the next unrelated glGetError() (e.g. read_pixel) then reports as failure.
- tests: gate the dlsym/libGL.so.1 fallback assertions on libGL being
  resolvable, so the GL suite also passes on a GLES-only host.

src/gl/loader.cpp

@@ -30,6 +30,10 @@ PLOT_GL_FUNCS(PLOT_GL_DEF)
 
 namespace {
 
+/// False until a `load()` has resolved every required entry point of the table
+/// above, which may stay partially populated until then. Read via available().
+bool g_table_ready = false;
+
 /// The system GL library, opened on the first name that neither the caller's
 /// resolver nor the global namespace can supply. Never closed: the entry
 /// points handed out must stay valid for the whole process.
@@ -138,7 +142,7 @@ bool query_caps(Caps& out) noexcept {
   out.persistent_map = storage;
 
   GLint profile = 0;
-  if (major >= 3) {
+  if (major > 3 || (major == 3 && minor >= 2)) {
     // GL_CONTEXT_PROFILE_MASK only exists from 3.2 on; older contexts would
     // raise GL_INVALID_ENUM and leave the value at 0.
     glGetIntegerv(GL_CONTEXT_PROFILE_MASK, &profile);
@@ -150,6 +154,9 @@ bool query_caps(Caps& out) noexcept {
 }  // namespace
 
 bool load(ProcLoader loader) noexcept {
+  // A partially resolved table must not be used: the fallible constructors
+  // consult available() and report Unsupported until the whole table is good.
+  g_table_ready = false;
   const char* missing = nullptr;
 
 #define PLOT_GL_RESOLVE(ret, name, args)               \
@@ -173,9 +180,12 @@ bool load(ProcLoader loader) noexcept {
     PLOT_ASSERT(false, missing);
     return false;
   }
+  g_table_ready = true;
   return true;
 }
 
+bool available() noexcept { return g_table_ready; }
+
 const Caps& caps() noexcept {
   if (!g_caps_valid) {
     if (query_caps(g_caps)) {