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/handle.cpp

@@ -1,5 +1,7 @@
 #include <plot/gl/handle.hpp>
 
+#include <plot/gl/loader.hpp>
+
 #include <cstdint>
 #include <cstdio>
 
@@ -78,6 +80,9 @@ GLuint alloc_texture(GLenum internal_format, GLenum format, u32 width, u32 heigh
 // ------------------------------------------------------------------- Buffer
 
 bool Buffer::create(Status* st) noexcept {
+  if (!available()) {
+    return fail(st, ErrorCode::Unsupported, kNoLoader);
+  }
   if (glGenBuffers == nullptr) {
     return fail(st, ErrorCode::Unsupported, kNoLoader);
   }
@@ -109,6 +114,9 @@ void Buffer::destroy(GLuint id) noexcept { glDeleteBuffers(1, &id); }
 // -------------------------------------------------------------- VertexArray
 
 bool VertexArray::create(Status* st) noexcept {
+  if (!available()) {
+    return fail(st, ErrorCode::Unsupported, kNoLoader);
+  }
   if (glGenVertexArrays == nullptr) {
     return fail(st, ErrorCode::Unsupported, kNoLoader);
   }
@@ -139,6 +147,10 @@ void VertexArray::destroy(GLuint id) noexcept { glDeleteVertexArrays(1, &id); }
 Program Program::from_source(const char* vertex_src, const char* fragment_src,
                              Status* st) noexcept {
   Program program;
+  if (!available()) {
+    fail(st, ErrorCode::Unsupported, kNoLoader);
+    return program;
+  }
   if (glCreateShader == nullptr || glCreateProgram == nullptr) {
     fail(st, ErrorCode::Unsupported, kNoLoader);
     return program;
@@ -219,6 +231,9 @@ void Program::destroy(GLuint id) noexcept { glDeleteProgram(id); }
 // ------------------------------------------------------------------ Texture
 
 bool Texture::create_rgba(u32 width, u32 height, Status* st) noexcept {
+  if (!available()) {
+    return fail(st, ErrorCode::Unsupported, kNoLoader);
+  }
   if (glGenTextures == nullptr) {
     return fail(st, ErrorCode::Unsupported, kNoLoader);
   }
@@ -235,6 +250,9 @@ bool Texture::create_rgba(u32 width, u32 height, Status* st) noexcept {
 }
 
 bool Texture::create_r8(u32 width, u32 height, Status* st) noexcept {
+  if (!available()) {
+    return fail(st, ErrorCode::Unsupported, kNoLoader);
+  }
   if (glGenTextures == nullptr) {
     return fail(st, ErrorCode::Unsupported, kNoLoader);
   }
@@ -280,6 +298,9 @@ void Texture::destroy(GLuint id) noexcept { glDeleteTextures(1, &id); }
 // -------------------------------------------------------------- Framebuffer
 
 bool Framebuffer::create(Status* st) noexcept {
+  if (!available()) {
+    return fail(st, ErrorCode::Unsupported, kNoLoader);
+  }
   if (glGenFramebuffers == nullptr) {
     return fail(st, ErrorCode::Unsupported, kNoLoader);
   }
@@ -310,6 +331,9 @@ void Framebuffer::destroy(GLuint id) noexcept { glDeleteFramebuffers(1, &id); }
 // -------------------------------------------------------------------- Fence
 
 bool Fence::create(Status* st) noexcept {
+  if (!available()) {
+    return fail(st, ErrorCode::Unsupported, kNoLoader);
+  }
   if (glFenceSync == nullptr) {
     return fail(st, ErrorCode::Unsupported, kNoLoader);
   }